Reach Classic Sandbox is a gameplay overhaul mod bringing Reach as close as it can possibly get to the gameplay of Halo: Combat Evolved.
This mod does not aim to include features from other Halo games, but rather, try and get Reach to be as consistent a prequel to CE as possible.
There is a Combat Evolved soundpack included with version 2 that overhauls the game's sounds to be more "classic" feeling.

This mod currently works for the Entire Reach Campaign + Lone Wolf.
V2 Changelog (see Readme.rtf for easier reading):
Global Changes:
Noble Team removed. Player character buffed to compensate (see below)
Combat Evolved soundpack added. Assault Rifle, Fuel Rod, Magnum, Plasma Grenades, Needler, Shotgun, Plasma Rifle and Sniper Rifle are all affected.
Specific Changes:
Player Spartan Stats:
    Damage resistance 200% (was 100%) due to removal of Noble Team.
    Health Regen Rate: 0% (was 100%, health damage now semi-permanent)
    Player Gravity: 75% (was 100%)
    Player Speed: 110% (was 100%) slightly increasing movement.
    Carried Grenade Count: 4 (was 2) Increasing carrying capacity for nades.
Equipment:
    Sprint:
    Active Use Drain Speed = -0.2 (was -0.25, sprint now lasts longer)
    Recharge Rate = 0.05 (was 5, sprint no longer recharges instantly)
    Recharge Delay = 1 (was 4, delay for sprint recharge significantly shorter)
Magnum:
    Accuracy buffed by about 25-30% from V1.
    Now functions very similar to CE counterpart.
Assault Rifle:
    Damage buffed to 6.4 per round. Was 6.
Plasma Pistol:
    Model reshaded to look unique, has a vibrant shiny blue somewhat similar to its CE     counterpart.
Plasma Rifle:
    Accuracy buffed by about 40% from V1.
    Given classic Plasma Rifle "blue" look. Has slight green fresnel shading to make it     look more like the Combat Evolved counterpart.
Plasma Repeater:
    Damage Buffed to 7.5 per round. Was 6.5.
    Model reshaded to a deep glowing purple. Suits the projectile colour more whilst     making it feel unique.
Needle Rifle:
    Rifle can no longer headshot. Damage is instead caused by repeated supercombine     hits as this weapon can break shields very easily.
Human Grenade:
Technical Changes:
The UNSC grenade has been completely changed to function like the "Mini-nuke" it     was in Combat Evolved. The following changes highlight this:
    Danger Radius (for AI): Reduced to 2. Was 2.5. (makes enemies less likely to dodge)
    Timer set to 0.5. Was 0.6-0.65. This makes it faster to detonate when armed.
    Set grenade arming to "on rest". This means the grenade must come to a standstill     before the grenade arms. (as in Combat Evolved)
    Damage Lower Bound is 120 (was 0), this is the minimum damage the grenade can     do within its radius.
    Damage Upper Bound now 235 (was 160), this is the maximum damage the grenade     can do within its radius.
    Splash Radius Min now 1.5 (was 0.6), this is the area for higher damage.
    Splash Radius Max now 2.5 (was 2.25), this is the area for the minimum damage.
    AOE core radius now 0.4 (was 0.54) this is the area for maximum damage.
The above changes make UNSC grenades harder to use (as they cannot be bounced easily like vanilla), but considerably more potent, especially when you can take advantage of a highly accurate throw.
The downside is that thanks to the need for the grenade to rest, this is harder to achieve, and the "core" radius is harder to hit than ever, requiring excellent timing (and luck) to hit the maximum damage.
Graphical Changes:
The UNSC grenade explosion has been made slightly bigger to reflect the new gameplay changes and make it similar in look to the CE grenades.
Plasma Grenade:
Technical Changes:
The Plasma Grenade has also been changed substantially to make it more inline with its CE counterpart. Below are the changes:
    Danger Radius (for AI): Reduced to 1.6. Was 2. (makes enemies less likely to dodge)
    Timer set to 2. (was 0.9) To behave like Combat Evolved Plasma Grenades.
    Damage Lower Bound is 120 (was 25), this is the minimum damage the grenade can     do within its radius.
    Damage Upper Bound now 275 (was 250), this is the maximum damage the grenade     can do within its radius.
    Splash Radius Min now 0.5 (was 0.54), this is the area for higher damage.
    Splash Radius Max now 2 (was 1.8), this is the area for the minimum damage.
    AOE core radius now 0.4 (was 0.54) this is the area for maximum damage.
This makes the Plasma Grenade considerably harder to use, but it has a larger splash radius as a result, as well as higher damage on direct hits. The longer timer can be confusing sometimes to enemy AI however, and they sometimes will step on grenades due to them not exploding quickly.
This makes it perform fairly similar to the Combat Evolved grenade, and as Plasma Grenades are generally less scarce than UNSC grenades somewhat less useful in certain situations.
However, as a direct hit grenade and general splash damage wise, it is more effective than vanilla.
Graphical Changes:
The Plasma Grenade has been updated graphically to look more like its Combat Evolved counterpart, with louder explosion and classic sound effects.
Rocket Launcher:
Technical Changes:
    No longer locks onto vehicles. (CE-like, makes it harder to use)
    Max Ammo carried: 10 (2 rounds in clip, 8 in backpack.)
    Max Ammo Stored: 8 (as above, 2 rounds in clip, 8 in backpack)
    Rate of Fire: 1 (was 0.8) this makes it fire slightly slower.
    Impact Damage: 300 (was 200) buffs direct hit damage.
    Splash Damage: 336 (was 240) buffs AOE, can now 2 shot scorpion and wraith.
    Splash Range: 1 to 3 (was 0.75 to 2.5) this makes the effect of damage larger.
Graphical Changes:
Given "classic" large explosion effect to match the classic style splash damage.
Vehicles:
Scorpion:
Functional Changes:
Change fire recovery time (ROF) to 4. (Was 1.75, to make it a more realistic "tank" and also more like CE)
Changed error angle (inaccuracy) to 2. (Was 0, this makes the weapon more inaccurate on the X and Y axis)
Changed AOE (splash radius) to 2 min, 4.5 max (was 0.75 to 1.75, makes the cannon considerably more powerful on a near miss)
Changed Damage to 400 (was 344, makes weapon much deadlier per shot, despite the large increase in ROF)
Projectile Changes:
Initial Velocity: 60 (was 100, makes projectile more believable speed wise)
Final Velocity: 40 (was 100, projectile slows slightly over time)
Air Gravity: 0.6 (was 0, projectile has realistic shell drop)
Graphical Changes:
Scorpion fires an actual shell model now. (previously was just an invisible projectile with a trail)
Scorpion shell has a new trail, much thicker, can be seen when you tilt your aim or see it shot from the side.
Scorpion shell explosion is now considerably bigger, mimiking the Scorpion High Explosive from Combat Evolved.
Falcon (New Alexandria Only):
Technical Changes:
Changed Velocity to 100 (was 400, if I recall, makes projectile tougher to aim and also realistic speed for a 20mm projectile)
Changed Damage per Round to 75 (was 20, gives rounds much more realistic damage vs infantry)
Changed Air Gravity to 1 (was 0, gives it a considerably more realistic shell drop for a 20mm cannon)
Changed AOE to 0.5 to 1.5 (was 0.25 to 0.75, to give it a more realistic splash radius for a 20mm APHE shell)
Changed Fire Recovery Time (ROF) to 0.33 (was 0.25, to account for the new power adjustments of the weapon)
Changed shots per fire to 1 (was 5, no longer a burst fire weapon, but a single shot heavy cannon)
Graphical Changes:
    Replaced Projectile Impact Effect (sparks) to a small grenade explosion (actual     20mm HE shells impact like this)
    Changed fire sound to shotgun fire sound (to give off the heavy-cannon effect).