OP, I used a lot of my FSA one year getting a fitness membership at the YMCA. You can prepay for any number of months in advance necessary to get to the dollar figure you need to use up.
be careful- your FSA administrator may require a letter of medical necessity to reimburse you for this. I think technically the services may need to be provided, so you could get into questionable territory pre-paying, too.
Someone mentioned massages- again, my FSA also requires an LMN for this, you might want to check with yours before going either of these routes.
There are some neat looking accupressure mats and pillows at fsastore.com. I had about $100 to use and stocked up on condoms, bandages, sunscreen, and random stuff like a light therapy device for acne and a little device that delivers heat to treat bug bites.
Love the idea to buy stuff and donate it!