When I threw my husbands 40th surprise party, we had 3 kids under the age of 3 and I was just pregnant with #4 so we had no money to spare! What I did was throw a 10(X4) kids birthday party. My husband never had a party when he was a kid so that was my starting base. Yes, I decorated the house with all the traditional kids stuff - balloons, crepe paper, silly hats, noise makers, etc I even hired a clown. Now, it did have an adult spin on it. The clown did more "adult" games that were just histerical and I also had a swamie who wandered the party and pretended to read your mind, palm or sometimes other places of the body. He was so funny and different that people loved it because nobody had ever did it. The entire party cost me about $300. The food we had were a couple kinds of mini sandwiches and lots of munchies. We had adult kool-aid (beer and mixed drinks) and just had a ball. Instead of regular gifts, I had the guests bring 40 of something that reminded them of my husband. Some where really creative and funny. For example: his brother brought 40 tylenol tablets because he was always such a pain in the neck when they were growing up but he also brought 40 pins because he was always the sharpest in the bunch.
I did it at my house so there was no major rental involved. We had a great time and people still comment about it 8 years later!