[Magic.]

My exs parents just didn't care so we could do anything at his house, overly lucky.
My parents were a different story. We were allowed the door shut but they would call up / knock on the door etc. We ended up having sex at mine sometimes anyway, the risk was kind of part of the fun ... And yes, I would assume that my parents knew, not that they would ever confront me about something so 'dirty and rude'.
Personally I would say hotel, my friend did it, and there are quite a few cheap hotels out there, maybe do a bit of internet research into hotels in your area [we didn't think there would be any in ours but there's loads]. I wouldn't recommend outside unless it's totally private [like your garden] just because from personal experience, it wasn't as good / exciting as was expected.