If you do like adventure travel, about five years ago, I went on a fabulous trip through
Wilderness Travel. They're not set up specifically for holidays for singletons, but to me that was a good thing -- I wasn't looking for a 'dating' trip, I just wanted a holiday where I didn't stick out like a sore thumb for being on my own. Their trips are amazing and are restricted in size so you really do get a lot out of everything. We had 12 people, 3 couples and the rest single people. They're US-based, but do trips all over the world.
Have fun! I'm jealous -- I love planning holidays!!