Red Dead. The main story missions in both games are great overall, but the optional side missions are a lot better in Red Dead.
I'd much rather do stuff break new horses and get into quick-draw battles than go bowling, play billiards, or drive to a diner (since I can do those things IRL.)
The fact that GTA is in its fourth 3D iteration kind of gives it a "been there, done that" feeling as well.
You'll need to figure out how to get yourself underneath a group of creatures hanging from the ceiling. When you're close enough, they'll grab and eat the worm. (You'll see a few of them fighting over one of the brainworms before you ever get one stuck in your head.)
When you have a worm in your head, you can only walk one direction, although walking into a patch of light will cause you to change directions.
There might be some sort of Just Cause game on the Wii, but a direct port of JC2 is unlikely. A good deal of the game's charm comes from the huge and detailed playing area, and the Wii just isn't physically capable of the same type of presentation.
Don't get me wrong, I'm not a Wii-hater. It's just that graphically intensive open-world sandbox games aren't ever going to be the console's strong suit.
I loved the first game as well. Played some at E3 this year, and it was a lot of fun. It's really story focused though, so if you prefer to skip cinematics and cause mayhem for the heck of it, you'll probably be better sticking with other sandbox-style games.
Unfortunately, although it has promise, it's got a lot of problems. The sniping part is pretty fun, but the stealth and enemy AI are sort of broken. Just wait until bad guys start shooting you with AK's from across the map, through dense foliage.
There's also way too much run-and-gun shooting for a game that's supposed to be about being a one-shot-one-kill "ghost warrior."
As you go through the main story missions, more weapons will become available to purchase at the various weapon shops. When browsing one of the stores, you can very easily tell the stats of each weapon, and how they compare to each other.
The main stuff you'll need to do is to complete all the main story missions, all of the "stranger" side-missions, complete all bounty missions, break all horses, complete all ten levels of each challenge (hunting, sharpshooter, treasure hunter, etc.). I don't even think that's a complete list.
The in-game menus should give you a better idea of what you still have left to do.