I think the best place to listen is a public camping ground. Common in Europe, but maybe rare in North America. The older campers often have camping trailers, but the young, student types use tents. and it's easy to hear even a whisper from 12 feet or so. I haven't any special listening equipment, but I've an older hearing aid that allows me to adjust the amplification up or down. Haven't tried it yet, but I think a small small plastic pipe from the hearing aid to the microphone opening on a mp3 player recorder would work well. Even better rig up a parabolic reflector from a wok, or such, to concentrate the sounds to the microphone. Funny incident was that I was surfing Youtube for sounds under the section: tent/ camping grounds and I'm pretty sure that one voyeur recording was of me and my wife!
I stay in hotels now and then while travelling and haven't much luck there. Cruising the hotel hallways is a definite NO No.
I don't want to get grabbed by a security guard and I would expect that nowadays many hotels have security cameras in the hallways. You could, if you dare, drill a little hole in the baseboard, carefully clean up the dust, and insert a small plastic pipe through the hole to the next room's wall. These little pipes carry sound very well and if plugged into a hearing aid would work really great. You could even fill in the drilled hole before you leave to hide the evidence. Best place to access the sounds from the next room is where there's an electrical outlet. The plastic cover of the outlet can be unscrewed and there's usually another outlet just across that allows easy listening.
Best listening I ever had was from a small workshop that I rented. A girl that I was really hot for lived above my workshop. She had a live in boyfriend so I didn't have any luck with her, but I found out that If I inserted a plastic pipe through a hole near the ceiling I could snake it up very close to their bed and hear every sound. The girl split up with the guy and he stayed in the apartment and really often he brought home teenage girls or slightly older women. Quite a stud and he provided real entertainment!