I just turned 40 and funds were an issue for me as well.
Just this past weekend -
I have a girlfriend in Houston and a couple of us met at her house. (she has a pool/hot-tub and heated it for us)
No hubbies or kiddos allowed.
We all brought wines, cheeses, fruit and chocolates. We made fresh guac and salsa. Everyone slept over for Friday and Saturday. We went Salsa dancing Saturday night.
One girlfriend brought a book about women and the different temperments - we had a blast reading it out loud and trying to identify ourselves.
Otherwise, we sat around and chatted, nibbled on great food, drank, watched chick flicks, and relaxed together (we all work demanding full time jobs).
So my suggestion - for fun and economy - find a girlfriend with a pool - willing to host, invite a few girlfriends and spend time together doing what you enjoy.
We decided the next time we get together (we do this twice a year) that we would hire a strong young fellow to come over and give us massages pool-side.
Happy Birthday - hope you have fun however you decide to celebrate!