Exposure

When I was a kid in secondary, I had a feeling this other boy had a crush on me so I rang him and asked if he wanted to come over, I then put a towel around myself - When he came to the door, I opened it and I said - Oh I was just about to have a bath I thought you knew I meant later and then I dropped the towel, I pretended to freeze in shock before picking the towel back up.

Here is an idea. Invite your friend over and hang out in the back yard. Accidentally spill something on yourself and exchange yourself to change. Go into the house and change in a room that he can see into from the yard. Have the window just enough blocked before hand that it seems you just don't know you are visible.