Places of historical interest (e.g. castles, cathedrals, etc)
Places of nature interest (e.g. national parks, forests)
Places of cultural interest (e.g. theatre, art)
Places to learn life skills (e.g. mechanics, electricians, cooks, tailors)
Places to have fun (e.g. theme park, sports games)
Places connected to subjects beings studied.
Trips I don't think schools should be going on - the ridiculously overpriced "voluntourism" holidays, where you have to pay a fortune to go an build a hut that local people would otherwise have been paid to build, where the "help" isn't really needed but the money is.