If you can get someone to keep you son over night, certainly take advantage of it. For my niece's b-day next month, all of us Aunts are taking her to the Improve. (It's her 21st!) We had talked about taking her to Pachanga, but we couldn't afford a room at $300 a night. We had also talked about renting a Limo to pick us up from the Improve. (We have a Van, so Hubby was going to drop us off.) Limos run about $65-$95 an hour, so that was kind of ruled out as well. We are planning to take to an Improve with a hotel within walking distance "just in-case". All of us are excited about it and have already taked about how fun it will be to make it a family tradition for the girls 21st b-days.
If leaving your son with someone over night isn't an option, then have a "Girls Night In". Rent your favorite movie, have each of the girls bring a Dinner course, (instead of gifts) and play you favorite game. It will be memorable, but inexpensive. (If you want to send your son away, you could still do this istead of going out.)
Last but not least, if you're brave enough, (I'm not) you could host one of those "Adult Toy Party" for your b-day. I'm certain it would both fun and memorable by all that attend. Just be prepared for some "Special" birthday gifts.
Enjoy your b-day! You deserve it. ~J