I had bookmarked this link for horseback riding lessons long ago, but never investigated it for more information. (If you find out the price/time of classes, I'd love to hear about it!) Here's the link...
http://www.escuelahipocampo.org/cabalgatas.htm
For tango, I think this website has a search tool for classes by day/location:
Tangodata
Suerte!
Steph