Help me plan my birthday party?

Hello, friends. ^_^
Okay, so here's the deal. My birthday is Feb. 28th, a month away, and I want to have a really fun party.
The theme I came up with is "Time Traveler's Convention." Everyone comes dressed as someone from their favorite historical era (medieval times, the 50's, the Old West, the future) or alternate timeline (anything steampunk, cyberpunk, etc). Famous time-traveling characters, like The Doctor or Marty McFly, would also be welcome.

What I need your help with is ideas for activities and decorations!

Keep in mind the following things:
1. I'm having the party at my house, and it's going to be cold and slushy, so we can't go outside very easily. However, I can use the family room and the garage and possibly the living room to spread people out, so I think we're okay for space indoors.

2. Low. Budget. I'm funding this from my tax refund, but it's still not a whole lot. I'm gonna have about $100-125 to spend on 15+ people (yes, it's a bit vague at the moment, I'll give more details when they come.) Now, I am planning to have people bring a dish to pass, and I'm making a main dish and dessert, so don't worry about food so much.

3. I'm turning 22, and my friends are all about the same age give or take a year or two. I need, for the most part, age-appropriate ideas. However, I am not opposed to a little silliness. I was thinking maybe we could all make marshmallow guns? I don't know, still planning.

4. Don't feel restricted to just things I've mentioned that I like! I'm willing to try new things and check out things I wouldn't have thought of. However...

5. I'm picky, so don't feel bad if I'm not feeling your idea, or if I decide to change it up a little. I have to keep everyone I'm inviting in mind, and you don't know all about every single friend of mine that's coming. Besides, maybe someone else will think your idea is sweet.

6. Side note: I work at a hardware store, and I get an employee discount. Bonus points if you can think of anything where I could get the materials there and save myself a little cash. Also, there's seriously giant cardboard boxes everywhere. I think I might attempt to build a time machine H.G. Wells style, if Mr. Wells had only refrigerator boxes, spray paint, and Christmas lights to work with.

If all else fails, we could get out Super Smash Bros or Mario Party or watch Bill and Ted's Excellent Adventure.

Thanks so much for reading all that, and even more thanks for your help!

xoxo
Apple