I used to do both ballet and figure skating when I was younger (ballet for 17 years and the occasional skating course between age 5 and 15) and have considered taking classes again as an adult, but haven't been able to fit it into my schedule (I work shifts and weekends and I'm about to go travelling and then move to a different city). I may consider it again after I've moved and started my new job though.
I know that our local ice rink offers both adult and children's group skating courses (30 minute lessons with about 15 people, lasting either 6 or 10 weeks... including free skate hire and you used to get a free 2-hour public skating session after the lesson too), but they're quite pricey from what I remember.
I'm not sure about adult ballet classes - I've thought about joining one, but I'm feeling rather inflexible and out of practice at the moment
. I also don't know what the standard would be like in an adult class (I used to attend the Bristol School of Dancing as a teenager, where they were pretty strict and you had to do everything perfectly or you were told off... I think I would feel out of place if it was an environment like that, but at the same time I wouldn't want to end up in a class of relative beginners).