Accidental or deliberate peeks are great, showing off her body on the beach is a huge turn on too, but what really gets me off is when my wife undresses where someone else can watch. She doesn't even need to take everything off - just the sheer suggestiveness of her taking off her clothes in front of a friend or stranger drives me crazy.
We were once out late at night walking back from a bar with some friends when we got caught in a torrential thunderstorm. We arrived back at the house soaked and dripping, and rather than ruining the carpet we all agreed to ***** off our outer clothes at the door. I caught the other guy watching Em as she slowly peeled off her wet clothes down to her underwear - which was also wet through and semi-transparent. It was getting a bit embarrassing as I had very little scope to hide my own excitement!
Stripping down to her bikini on the beach, changing into a party dress in a friend's car, taking off her top at the piercing parlour - I just love it when she undresses.