I used to travel a lot on my own before I was married, and still do travel alone occasionally. I like taking "language study" holidays and find that they're a great way to meet like-minded people, add some structure to my day (but not too much) and link up to sightseeing activities through the schools I'm attending. They can also often be relatively inexpensive travel options, because language schools usually offer homestay and other inexpensive accommodation options (although I prefer to stay in hotels). Depending on how important the study part of the trip is, I'll sign up for 2-8 hours of classes per day for a week or two. I usually find that at least one of my classmates is someone I'm interested in sightseeing with. The school also usually organizes after-class and weekend activities. I've taken courses in Strasbourg, Grenoble, in a mansion in the south of France, Salamanca, Madrid and Barcelona. Along the way, I've met interesting people from all over the world (including Mongolia, Libya, Cuba, etc).

If you're not into languages, you could take the same approach but focus on something that interests you (e.g. cooking, fine arts, motorcycle maintenance, etc). Plan a trip that includes a course at the outset - and then travel afterward.