The Stack/Aloft is pretty much casual. Except for, as the PP said, Homecoming night (only in The Stack on the Magic), and even then, most people change out of their semi-formal clothes back into shorts/jeans, etc. after the first hour. I've worn everything from shorts and t-shirts and tank tops, to bathing suits and cover-ups, to jeans and sweatshirts, to sundresses. Pretty much anything goes. There isn't a dance party
every night, and when their is, heels and cocktail dresses certainly aren't required (or even recommended). So don't sweat it at all.
As for activities... I remember making flubber only on one cruise since I've been in the lab. And during that, we went down to the lab at about midnight and did it. It wasn't during the day because you'd then be using up the facilities of the younger kids. Other activities (Gotcha, video scavenger hunt, gender wars, brains v. braun, glow jam, etc...) take place during the day. There's a system (or at least there was on my last sailing... not sure if it's still in place but it was new then) called Torch, where the counsellors let you pick out your own activities to schedule in the navigator. The Stack/Aloft is a blast. Have fun.