I posted this same question last spring and got some terrific ideas from TE'ers. I sifted through the possibilities and ended up doing a week-long bike tour of the Natchez Trace with WomanTours in late April.

The trip wasn't cheap but was worth it. I had a ball spending my 50th birthday with about 20 feisty, friendly, funny women from all over the country, all levels of cycling skill. The 226 miles I logged that week were the most physically challenging thing I'd ever done---including the 50 miles I logged ON my 50th. For me, the trip was a perfect way to celebrate the big milestone.