The most important activity is...
The most important activity is people watching. I'm a photographer also (see johnnyriggs.com) and love watching faces go by, taking photographs, etc. Spend a lot of time in town, walking Commercial St.,
soaking up the local color. My fondest memory is almost always from the previous year. I love getting on my bike with my honey and riding into town. Immediately cramming a lobster in my craw.
If your into scavaging through the remains of what was left when the tide was out I recomend sandles. The reson being that if you wear aquasocks the wet ground sucks them up like quicksand and your foot gets stuck. Then when you pull your foot out you lose your sock, all gone, by-by, it's crab property now. if you have sandles with straps around your foot they won't be pulled off, plus its easier to get all of the rocks out too.
Consider the "off-season"
Consider visiting in the winter if you're relatively close. There is a certain charm and remoteness to the Cape in Winter----and it's not always snowbound. It depends what you like. The sea breezes can be bitter, but on a calm day a walk on the beach is beautiful and quiet.
The sand here is very grainy--mostly made of the tinniest little rocks you've seen. This makes for a good foot massage while you walk barefoot along the shore! This here is called Race Point Beach--fairly deserted!
A 242 foot monument to the Pilgrims. You can climb up to the top of this thing. It has a good panaramic view of the entire Cape Cod and some areas across the Bay around Plymouth. I personally did not go up in this thing.