Hmm, maybe you could get a photo in front of the entrances to each of the lands or in front of the 'mountains'.
I'm with lalasmama on the princess suggestion. What teenage boy doesn't want to get close to a pretty princess :good vibes
On that note, finding them with as many characters as you can might be interesting as a collage, but would take a lot of time to do.
Since they are older, you could look through your past photos, find one when they were younger and try to replicate it. You could put the two side by side and see how both your munchkins and the parks have changed.