Cast Away/Headscratchers

He buried Albert's body fully clothed
All that fabric, the wallet with plastic credit cards, and Al's belt would have all been pretty helpful.
 * I'm guessing at the time he didnt think to pick pocketing a corpse, or wanting to see a deadman's junk.
 * Also, at that point in the movie he hadn't settled into his Robinson Crusoe persona yet. He wasn't thinking about what he'd need to survive, he was just flailing around trying to think of something to do. By the time he learned how to forage properly and realized how useful Albert's clothes would be they would have been pretty stanky, and probably not safe for him to handle.

The seafood buffet on his return
Good heavens - don't you think this guy had had enough seafood? He probably wanted a burger!
 * It's been a while since I watched the movie, but wasn't that the joke?
 * Personally, I thought the joke was that the crab legs at the buffet were so much bigger than the crab he'd been eating on the island. The look on his face, to me, seemed to say, "Where the hell were these giant crabs on my island? Did I get stuck on the Island of Gimpy Crabs?"
 * I think the writers knew that added that for Rule of Funny.

The package he never opened
Why did he never open that package during the 4 years he was stuck on that island? Wouldn't he want to know if there was anything useful or potentially life-saving in it? Also it seems rather absurd that after 4 years that he would give the damn thing back to the address it belonged to, you would think that they would have long since forgotten about the item they had purchased if they had not already replaced it.
 * I've seen that as a "the mail must get through"-attitude, an intentional unfinished business that would serve to get him through low spells. Also there are occasional reports about letters delivered with a delay of decades, and the reaction is generally not "why do you bother me with this now".
 * A FedEx commercial for the super bowl parodying it has the final scene of the movie be a little different where the person the package was addressed to meets him at the door and accepts it. Out of curiosity he asks the woman he meets to open it for him so that he can know what was in the package. It was a satellite phone, a GPS, fishing pole, water purified, and seeds. Had he opened the package he could have gotten off 4 years sooner.
 * Well, according to the third draft of the movie, on the 1,000th day on the island Chuck opens the package which contained . More info here.

Why? Why did Wilson have to float on?

 * Why couldn't they make it so that he didn't? My God, that was the saddest part of the movie. The saddest part of any movie ever, at least for me.
 * It says something for Tom Hanks's acting skills that he can make us teary watching a man losing a volleyball.
 * The fact that it's so sad is the whole point. What were you expecting? That he would wash up on dry land and spend the rest of his life talking to that damn volleyball? That would've totally killed the mood of the movie.
 * "Kelly, I'm back, I love you, and this is my volleyball: Wilson!"

The plane crash

 * Why does the engine he sees nearby in the water just after the crash seem to spontaneously burst into flames and explode? Maybe some airplane guys can shed some light.
 * I'm not an airplane guy but I would guess it had something to do with the water being sucked into the engine and...something to do with pressure in the engine and...bad things happened inside and it 'asploded. Yeah.

The suicide dummy
Why does the fact that the test rope was too long mean he couldn't hang himself? Surely finding that out was the point of testing the length in the first place?
 * The rope wasn't too long. The tree he tied it to broke in half when he dropped the dummy off the cliff. If Chuck (who presumably weighed a lot more than the dummy) had tried to hang himself from that tree it probably would have snapped the tree right off and he would have fallen onto the rocks below, where he could have broken his legs or his back (or both). And without any way to get back to his shelter he would have suffered a slow and torturous death from exposure. Afterwards he was too spooked to go back up there again.