We had a Superhero bday party for our son on his 5th bday and it was a blast!! Here are some of the things we did....
We decorated with balloons and streamer in blue, yellow and red (Superman and Batman and Iron Man colors).
We bought little gift bags and cheap giveaways with superhero themes (batman bracelet, iron man pencil and notepad, etc). You can find lots of reasonably priced things at the dollar store or party store. We also put a couple of suckers in there cuz we had a TON of suckers left over from something else we had done recently.
I made a cape for each child using a piece of fabric, iron on letter (their first initial, and ribbon. I did not sew at all! Only ironed!
We made a Superhero headquarters on the playset in the backyard - with costumes (my son already had a ton of them), pretend laptop computer, binoculars, pencil and paper, etc (any spy-type gear we could find).
The kids completed a Superhero training obstacle course - lept a tall building (blocks) in a single bound, practiced flying (on belly on swings), found kryptonite in sandbox (green block to be removed with tongs), rescued a baby (doll) trapped in a tent, and drove the batcycle (big wheel). We had one at a time go and timed them. It was superfun!!!
Soon the Birthday Basher came! He is a super villan, of course. He dropped bombs (black balloons) on the party and the kids had to de-activate them (pop them with feet) before they blew! Then I handed out silly string very quickly to each child so they could stop the Birthday Basher (my husband dressed in tight, bright clothing and a ski mask!). They chased him away with the silly string and saved the party!
We had a kryptonite cake - white cake with holes poked in center and warm green jello poured over top, cooled in fridge, then frosted with cool whip! Yum!!
It was a blast! We were lucky enough to have beautiful weather for the party in April, but lots of these things could be done inside as well.
Good luck! Have fun! I love thinking of fun things to do at home for birthday parties. :o)