There are lots of things going on during it but sort of there are five main activities you can do at these.
First is you can look around the dealers area and buy stuff. The dealer shops are pretty varied and have all sorts of stuff (comic books, tee shirts, toys, games, art, pretty much anything nerd related). That is a pretty big part, the dealer floor is huge and takes hours if you go through and look at all the stuff.
Second you can go to panels. They will have a massive schedule of stuff you can see panels on, it can be like specific tv shows or like star wars or DIY stuff, its pretty varied. These are also your chance to see the celebrities talk for free (well not free but not for any extra money other than your admission price). All the big celebrities will have at least one panel they will do which will be them talking and a lot of Q&A.
Third you can play games. This can be like board games, or table top games, or video games. I've never really done this so i'm not sure exactly how it works but generally a lot of people are just playing games the whole time. They generally have some sort of video game tournament as well.
Fourth you can buy autographs / pictures with celebrities. The celebrity autographs/pictures pretty much always cost extra and can be expensive. some of the photo ops can be like over a hundred dollars and most autographs are between 30 and 75 dollars each.
Lastly you can people watch and check out all the people in costumes. There will be a lot of people in costumes and they will just be roaming around the entire convention. Some are amazing and some are terrible, the people are pretty nice and you can generally take their picture / take pictures with them if you ask. They will also have a costume contest at some point and you can see the best of the best at that (it'll just be like a panel and not cost extra).
It's a pretty good time, the people watching is worth the price of admission alone. you may not be into comic books but there is a huge variety of nerd things there so you'd be hard pressed not to find a shop selling something that you nerd out on.
