We did Berlin by Train (just the train) a year ago. The train didn't have AC, but you could open the windows, so it was OK, even with near-record heat that day. I believe the train had bathrooms, but we didn't use them. We got snacks going and coming back. We went the DIY route and spent the day at the zoo, which is outstanding. It was a Monday, and all the museums were closed, which took some people by surprise.