I’m confused by this. Mario Kart’s multiplayer is just as “simultaneous” as Wii Sports Tennis/Baseball is, the only difference is that you’re playing on your own screen segment, as the game wouldn’t really work any other way. You’re still racing against the others in a single environment-it’s actually just like the Wii Sport Boxing game, if you’ve tried that. It sounds like you just may not be a fan of “split-screen” gaming.
Anyways, there are a few other games you may want to consider, that are not “split-screen:”
Samba De Amigo: A hilarious and very fun (and tiring!) music game where you have to shake the controllers (maracas) in time with the music–it’s much more fun than it sounds. Try it out–has awesome music and is only $30, I believe. It maxes out at 2-Players though.
Super Smash Brothers Brawl: An awesome party/fighting game that’s pretty easy to get into. Everyone battles it out on the same screen, and it’s some of the most fun I’ve ever had. But if you’re new to gaming, and don’t have fast reflexes, it may be a bit too advanced.
Mario Galaxy: This is more of a love/hate type thing. It’s primarily a single-player game (and an amazing one at that), but it does feature an optional cooperative mode where the single player controls a cursor on screen and can help the other by collecting stuff, blasting enemies, and doing other things. Cool, but may be an acquired taste.
Boom Blox: I second the poster above, this is a very fun game that 4-people can play in both competitive and cooperative modes. It’s very easy to play and quite fun, but it’s not “simultaneous,” as the players do take turns playing, like Wii Sports Golf.
Wii Play: This $50 game is actually 10-games (or thereabouts) in one and comes with a Wii Remote, so in effect, the game only costs $10 (assuming you need the Wii Remote). The game’s not anything too special, but it does have a couple of games on there (laser hockey, and tanks!) which are 2-player and pretty fun. But I would suggest this if you need another controller, as the game itself isn’t worth the asking price.