We have an "only" -- a ten-year-old boy -- and have found that active vacations work really well. Generally, we do cycling or multisport (hiking, rafting, kayaking) vacations with active tour companies (REI, Backroads, Butterfield & Robinson, etc.). My son loves being part of a group of kids, while still doing stuff with my husband and me. Because the adults are doing the same thing the kids are (hiking, cycling, etc.), there's no "kids stuff" stigma to the activity. Usually, there are kids-only activities in the afternoons that the teens opt in or out of, depending on their interests/maturity.
Because these are active vacations, there are usually more boys than girls on the trips. If you're interested, look at the different websites, trips, dates, etc., and call the companies re. the ones you're interested in. We usually try to pick a trip that has kids around my son's age and older. Ones with a large cadre of 6-8 year olds and no other teens would not be good.
Good luck finding the right solution!