I strongly recommend U3A. You can join as many or as few groups as you like - I belong to a philosophy and a history group, but I would like to go to the Shakespeare meetings if they didn't clash with my other commitments. The beauty of U3A is that everyone learns together and everyone can contribute. Each group has a convener, to manage the admin, but there is no teacher. Basically, everyone is a teacher and everyone is a student. In my history group, for instance, we concentrate on the Middle Ages and everyone chooses an area of interest to research. (Thank God for the Internet!) Then we report back to the group and there's a general discussion. It's very relaxed and enjoyable, with everyone's contribution being valued, and I've learned such lot.
Each U3A has different groups on offer, but there's always a wide range of subjects and, if you're interested in something which isn't already offered, you can always suggest it to the Group Co-ordinator. If there's enough interest - and groups can really be quite small - they'll help you to get going.