Interesting, it seems like no one has answered this yet. Well, although I have no ability to run the PC version of UP right now (I used to have it, but sold it off a long time ago), I have played that version a lot, so I can give some info on the collectibles of the game.

So throughout the game, there are pictures that Carl has to pick up and butterflies that Russel has to pick up (and they HAVE to be picked up by the respective character as well). However, not all the pictures and butterflies will be available right from the get co in all levels. Sometimes you get an upgrade that allows you to progress throughout later levels, but you also have to go back to previous levels with those upgrades as well to get all the pictures and butterflies.

Next, there are also these bugs in all sorts of places around the level that pop up. You have to win the bug competition that pops up. If you're playing Solo, then the AI for the most part will automatically go toward things that have bugs in them, such as certain rocks, rotten logs, rotten fruits, or even just in holes that are around the levels (bugs come automatically out of those holes, after which the AI will start to collect the bugs, but the other objects require an input to break open and pop bugs out of). You have to win that competition in every level (if you're playing co-op, you literally just need to collect or squish one bug and then finish the level normally, since there is no AI that automatically goes toward those bugs, however the AI won't open anything that has bugs in it if you walk far enough past it).

Finally, there is actually a time limit for every level. For whatever reason, for 100 %, you also have to complete the level within a certain amount of time (that time isn't seen until the results screen after finishing the level). This time is called the "Best" time. Of course, for a speedrun, the first playthrough should be the fastest so that you don't have to worry about most of the collectibles (I would make an exception for the bug collecting, though, that should get done along with the time limit) since the photos and butterflies are the collectibles you won't be able to get all of until after you've got everything (and you basically have to complete the level again to get everything anyway, so it's not like you have a choice in coming back even if you do collect some of the pictures and butterflies on your first attempt of the level).

And that should be the gist of the collectibles for 100 %. I might have missed a detail or two, but you can keep asking me questions in the forum if you need to.

You can also feel free to ask me what I think would be fastest in terms of collecting things.

Yeah, I don't know about that. I didn't play the Wii version.

Yeah, no such leveling up exists on the PC/PS2/PSP version.