I'd go around terraforming barren planets to be full of life and such. That's so they could see my ultimate masterpiece; Where I align the stars to look like a giant chi fist punching a dragon viewable from any planet in the known universe.
Being a little more ambitious, and remaining within the Marvel 616 Universe, I'd be Surfer - just explore the whole universe in a few seconds, then goof off for the day with his powers. Meld with a star for an hour, disperse my consciousness across a planet (maybe across a hot women's synchronised swim team), create weird elements and oh yeah, KO Thor just for the hell of it.
Ultimately, it'd be insanely thrilling to be Hulk for a day, without the guilt and consequences. Annihilate Rulk and the military, permanently kill off The Illuminati (well, Reed, Stark and Xavier really) and then kill Thor just to prove to him that he never really had a chance back in the day (if he thinks his luck is bad now, he should remember the savage KOing he got), nevermind now that Hulk doesn't have the internal conflict going on. Oh and then I'd sign over a bunch of BannerTech patents to me ... not sure how well IP can transcend the fourth wall, but it's worth a shot.
Actually no; at least, not primarily. I picked him because I like the idea of a younger, rawer character. He's not at that whole "apple pie" stage yet.
I like that whole "I'm taking on an army of Parademons, everyone thinks I'm awesome and I don't give a ****" aspect we saw of him in JL.
...something something time manipulation loophole.
Which is negated by making it a "relative" day, regardless of time shenanigans.
Then it probably doesn't matter. If any of us picked a relatively powerful power set, we'd spend the whole day either trying not to die or trying not to kill anyone by accident. I'd pick Batman and assume it includes his riches. Then I'd buy a bunch of awesome stuff, hoping it stayed with me after the day was over.
If it didn't, I'd just call a press conference with a "really important announcement." Then I'd fart into the microphone and laugh uncontrollably, before high-fiving my bikini-clad assistant.
you're in a comic universe for "a day", why would you care about killing people?
also, TOAA transcends the comic - being representative of the creator/company behind the comic - and reaches right into our real world. So, yeah ... I'd just hire myself to be the Chairman for Life, of Marvel.
I'd still chose Surfer. And maybe I'd go into DCU and hook-up with Power Girl, Wonder Woman and - given my indifference to morality in comics - Supergirl!
Oh and ofcourse I'd kill Superman, Lex Luthor, Toy Man etc etc... but that's only the morning.