English day trips are great with both adults and children. These trips are great opportunities to learn about new things and learn new vocabulary that they would not necessarily learn in the classroom. Going to the museum is a great idea but you should be well prepared. Plus, you have to prepare your students and explain properly what you expect from them. Leisure is to be avoided and they should have opportunities to speak English as well. I would suggest doing a guided tour of a specific place in town. You could encourage your students by telling them to ask a question to the guide. I also love the idea of the Treasure Hunt. A lot of foreign students do this activity when they are visiting a new town. My collegues suggested a lot of excellent ideas about tasks to do during an English day trip. You can use those as well.