Cosplay is a bit more involved than you may originally think. It's not just a matter of looking like the character, a good cosplayer has to be very knowledgeable about the character. Lots of time and effort go into making the costume, as well.
Cosplay, however, is not the only thing you'll find at an anime convention. In fact, there's actually a real rift many of the people that cosplay and many of those that don't.
Video games are a big draw at anime conventions. Alas, I can only say good luck in trying to get your hands on a game if you're going there to play. You'll find the extremely popular games like DDR and Para Para and you may see some old school favorites as well. Unfortunately, you're also likely to find the same group of kids cycling through the same game over and over and over again (I have yet to be able to sit down and play Halo, and I've been attending anime conventions for 4 years).
Non-video games also have their place. You can usually find a group of Yu-Gi-Oh or MTG players hanging around as well as an RPG or two. There is almost always some sort of LARP as well.
The two types of discussion panels you'll find are industry and fan based panels. Industry panels are panels run by companies. This is where you go to find out what's going on with any particular company (ADV, for example). These are highly attended as it is well known that the panelists tend to hand out prizes to audience members that know their trivia.
Fan run panels cover everything from guest of honor speeches right down to individual items such as kimonos, video games, Japanese language, voice acting tips, AMV making tips, and many more.
There may be workshops in which you can learn how to construct and/or paint a mecha miniature model.
In larger anime conventions, you will almost always find autograph sessions with the guests of honor. Show up early to get in line or you may not make your way to the front to get the autograph.
Game shows are also popular at anime conventions. These are based on your favorite television game shows but the questions are all based on anime.
Dealer's rooms will vary in size depending on the size of the convention. The larger the convention, the more variety you'll have in what's available. You can find anime, manga, CDs, clothing, backpacks, bags, action figures, statues, toys, wallscrolls, posters, and all manner of collectibles, and many of them straight from Japan.
And, of course, what would an anime convention be without anime. The amount of viewing rooms depends on the size of the convention. Larger conventions will have at least three rooms, each showing a variety of anime.
Just remember, anime isn't just for kids. Adults do attend as well, and they have just as much fun reminiscing about the older anime as the kids do talking about the new anime.
