For my twins' third birthday party, we did a treasure hunt with photographs as the clues. So, the first clue was a photo of the bookshelf, and in the bookshelf was a picture of one of our plants, and in the plant pot was a picture of the bathtub, and on and on. The kids loved it. It might be too boring for your older guests (for the fourth birthday we did a hunt with written clues), but it's great fun for the 3-and-under crowd. For a treasure at the end, one year we had playsilks which I had bought in bulk as blanks and dyed with koolaid, and another year we had polished stones.
Other fun ideas: we always make a piñata out of paper mache and put little animals in it. For the cupcakes, one year I made a couple of caterpillars by stringing them all together, and putting faces on the front cupcakes. This year we just used cocktail umbrellas as the cupcake decoration, with a sliced strawberry lying underneath, "at the beach."
I like to try to make the parties have as little waste as possible, so we use real dishes and cups and silverware (we just make sure our dishwasher is empty beforehand). And I don't want to send our guests home with anything that I wouldn't want lying around my house. So, every year dw makes little bags out of leftover fabric scraps for the "goody bags," and for the goodies we put in stuff like a mix cd of the kids' favorite music, a mini notebook and crayons, and an organic fruit leather treat.
I do have fun doing the birthday party thing!