LPT: If you see 2 people together, one taking a picture of the other -- offer taking a picture of them both together.

Chances are they appreciate a helping hand. Came across a mom today with her lil daughter, mom was taking a picture with her iphone. Offered [see title], kid emberassed, mom happy.

Bonus: They probably will remember you whenever they see the photograph. Do everyone a favor, be selfish sometimes. :p