When my son turned 5, he was into super heroes, bikes, and legos. So we invited everyone to dress up like their favorite super hero, bring their bikes, and we went on a Super Hero adventure. Our driveway was turned into an obstacle course for the trikes and bikes. We decorated cardboad boxes as city buildings, and I even bought a few of the orange safety cones to drive around. My husband even build a ramp / bridge to drive up and over.
Then they were told that a dangerous villain had attacked the city, and they needed help. So we went into the playroom where a set of legos were on the table. Buildings, etc. had been made on a base, but "sustained damage from the villain", so they had to "repair" the buildings and set it back up on the base. I had prepared a partial building for each child (to save some time and give some guidelines).
Then we had to hunt for the villain, so they had to follow clues, like a treasure hunt. I drew pictures to go along with the clue that was read to them which took them all over the yard, to finally where the "dragon" pinata was buried under a pile of sand (in a plastic bag, so it wouldn't get ruined). They had to DIG it out of it's cave, then, of course, we strung it up and used the pinata stick.
The kids saved the day !!! They had a blast ! And there were only a few things to buy - orange cones, wood for the ramp, LEGOS - (which he was getting as a gift anyways), and a pinata filled with more little toys than candy. OH, and a few CDs that had SUPER HERO music on it (Superman, Batman, Indiana Jones, etc.).
Two girls attended that did not previously have that type of costume. Their mom printed out a computer iron-on that said "Super ____" with their name on it. That would be a pretty fun craft to do for all the kids. They could bring their own cotton shirt for the iron on, and would be considered a great "goodie" to go home with, esp. if a cool small cape was safety pinned onto the shoulders.