My DM has taken her 20 family members (her four children, grandchildren, spouses, and great- grandchild) on three cruises now. The best thank-you gifts for her are not things. Her favorite part of cruising is all boarding together and getting that photo taken when we board - the smiles and happiness of being together - each family group coming from a different part of the country - are amazing. She keeps the photos on her refrigerator.
Someone always does matching t-shirts (or polar fleece vests for Alaska) and we wear them when boarding. We always eat dinner together and each day sit at different places so different grandchildren get to sit next to Grandma. Invite Grandma to lunch in the dining room with only a few people so you can talk. Flowers in the room for embarkation are appreciated. Then, after the cruise, each person writes and tells about their favorite times together - some send a top ten list. And send photos. Everyone so enjoyed this past Thanksgiving on the Wonder that she immediately booked another Thanksgiving for us all.