Elderhostels!
If you don't know what they are, check
www.elderhostel.orgThey offer an amazing assortment of trips all over the world, all of which are geared toward learning as well as having fun in a new place. Most are scheduled slightly off-season so the rates are kept low, and they use local experts so you get good information and guidance. While I often travel alone, it is really nice to have that local knowledge when I am exploring a new area.
I have taken 16 Elderhostels so far, and am eagerly awaiting my next one, birding in the Grand Teton national park in Wyoming. I usually take outdoors courses and they are rated for physical activity on a 1-5 scale, so you know what to expect. The people who take Elderhostels are usually interesting, active seniors who are a delight to be with.