There are lots and lots, believe it! If I'd seen this thread sooner I would've responded but it came up while I was traveling lol.
Anyway, the games that are playable cover a wide variety of genres from fighting to sports to rpgs. So here are a few that come to mind starting with gameboy and gameboy color.
The obvious ones are the main line of pokemon games from generations 1 and 2,, being red blue yellow, and gold silver crystal.
My favorite racer and the only playable one I know of on the gameboy is f1 race. It is a lot like top speed 3.
Next is battle ping pong, which has different sounds for different ways to hit the ball, and you dont need to worry about having to move the paddle left and right, just a to serve, b to hit, and up and down plus b to hit your ball differently.
There is a nice box pushing puzzle game called kwarx I think it is that's very playable because of its sound design.
Battle ship is also playable, complete with a lengthy campaign, leveling system and weapon upgrades. Another playable game is radar mission, but only one of the modes.
I haven't mentioned any fighters because there are all the obvious ones from sf to mk to ki to king of fighters to fatal fury to super Chinese fighters. Ok that last one might be a little mor obscure.
There are the double dragon games if you want beat em ups, surprisingly those are a little more rare on the gameboy. The first tmnt game is another one but not the last two.
If you're using emulation then the game and watch gallery games are a fun diversion of minigames, many of which are playable, with more to unlock.
Lots of shooters too if you dont mind the use of emulation to help. Space invaders actually has stereo sounds.
Q'bert is also playable, but of course you wont be able to hear the enemy so it'll take trial and error. Lock and chase might also be playable eith some help.
Track and field is mostly playable, ESPN's new international track and field is more of the same. If you have a way to read the text then the yugioh games will probably be playable.
Obviously there are tons more. Due to the simple nature of the gameboy and gameboy color there is a lot less you need to worry about so it's really just a matter of taking the time to experiment and figure out what you like. I will leave you with two more gameboy games, bionic commander which is a 3d mech combat game, and side pocket if you're up for some billiards action. O also kwirx also has a sequel if you like it but I dont remember the name. In my gameboy and gameboy color collection on my psp, I have about 90 games that I would consider at least somewhat playable, and at most completeable.
On the gamegear side of things, it's a little less complicated because of its smaller library. The standout playable games on gamegear are the two power rangers games, both streets of rage games, and virtua fighters mini. Also double dragon. Of course there may be more that I dont know about.
Hope that helps. That is quite a lot of info i know, but I'm sure it only just scratches the surface of what we can actually play. If you want to include gameboy advance, that's a whole another can of worms to sort through, but if interested I will list my findings for gba too.