I've only been to Miami during WMC season... which is fucking GREAT if you're into dance music, clubs, partying... but right now, I don't think there's too much going on. Eat Cuban food, avoid any other kind of food, walk around SoBe, hang out at the beach.
To be honest, even though I live near I have never been to the Guggenheim

MoMA, the Met, Museum of Natural History are all recommended though.