In your Navigator there will be a symbol next to the activities that have cards available. Some of the biggies are the evening shows in the Walt Disney Theater (Hercules, Dreams, Welcome Aboard Variety Show, Farwell Show, Mouseketter). There are other activities like the Island Magic show in the Buena Vista Theater. We receive a couple from our room steward and I think we got one (autographed) at one of the Captian meetings.
Just be sure to check you Navigator! And yes they are great with your picture album or scrapbook.