Too many fun things to do with a Action Replay. However, don't use the cheats to finish the game if you haven't done so before. It'll ruin the fun.
Anyways, think of the things you can do!
Wind Waker: Fly anywhere with Deku Leaf, and have fun control(means you can ascend too!), and fly faster than boat can sail! Also, superfast running/swimming, endless swimming, diving, and the main moonjump, that also works with Deku Leaf to let you ascend.
Metroid Prime 1/2: Moonjump, rapid missiles, rapid power bombs, endless ammo, rapid beam combos. no need to charge your beam, all kinds of 1P exclusive moves for multiplayer(such as screw attack and beam combos)
Mario Sunshine: Superspeed run, DX-long spinjump, moonjump, Yoshi swimming, running underwater, wall run, surf anywhere, being Shadow Mario, Mario-recolored to be Luigi, and having green Yoshi.
SADX: Endless maxspeed boost move(See, this is how fast I want a Sonic game to be), moonjump, Super Sonic anywhere, Metal Sonic anywhere, Shadow-like Sonic, any character in any level, and run on any wall.
SA2B: Endless maxspeed boost move(YAHOO!), any character in any level(including multiplayer characters), Super Sonic and Super Shadow anywhere, moonjump, and fake-Super Sonic.
Super Smash Bros Melee: Too...many...awesome things. Rapid charge beam, ANY level(including 1P and test levels), debug menu, 6 man melee, ANY character(Including Master hand and Giga Bowser), hidden gamemodes, extra moves and abilities for characters, move swap, Ganondorf w/sword, oh my god, I can think of waaaay too many things!
Well, there many more of em, but I'm too lazy to say what they all have in store for you.