Education is always an option. Either by distance learning or local campus. What interests you. What have you always wanted to learn about. Everybody has their own thing but there is lots to choose from. When I semi-retired 11 years ago I spoke two languages and was iffy on history and geography. Now I speak read and write comfortably in 4 languages and continue to be fascinated by history, culture language and geography. (really all four tie together nicely). I am a wealth of completely useless information. :) The internet is a huge resource for the on the road full timer bunch like us. Carrying on a conversation with someone in Germany, Costa Rica or French Polynesia is literally a mouse click away. Ask yourself what interests you, do a little digging and see what resources are out there.
Enjoy and happy trails.