We did a camping theme for my daughter's fifth birthday. We made invitations to look like camp brochures using the publishing program on my computer (even had one mom think it was a real camp!).  We had the party at a park (a few hours in the afternoon - not ready for a sleepover at this age!).  We borrowed tents from friends (and a few people brought with them when they came, too) and pitched them around a cellophane campfire (red and yellow cellophane mounted in cardboard, surrounded by rocks to make a fire circle).  The kids made paperbag backpacks and starry nights (toilet paper rolls with black paper over one end with holes poked in it), went fishing (magnetic fish in a baby pool), fed a hungry bear (threw plastic food into hole cut for the mouth in a big picture of a bear mounted on a big box), played in the tents (had them bring camping gear, so there were sleeping bags, lanterns, etc.), and played on the park's playground.  I had a bunch of organized activities planned (like singing around the campfire, some games, etc.), but never even got to them!  The party wasn't over a mealtime, so for food we just had chips and a veggie tray for munchies, then the cake was designed like a backpack, using fruit leather for the straps and red licorice strings for outlining the edges and flaps. Of course, it was served with S'mores ice cream. Goody bags were brown paper bags with bug collecting kits, compasses, and bags of trail mix.   For thank you notes, I cut and folded construction paper to look like little backpacks, with the child's name on a "label" on the outside flap, which then opened up to read the note inside. The kids had a blast, the parents enjoyed themselves, and the party pretty much ran itself (which, with 17 5-year olds, is a good thing!).