My favorite relationship books include:
The Harville Hendrix book gave me the idea for this website as one way to give a relationship a boost. He suggests couples create wishlists for each other to use weekly. Every good relationship takes effort on both parties to ask and receive as well as discover and give.
With a group, go in together to buy someone something special, plan some water fun,
play a board game like Apples to Apples or Boggle.
Or do something phenomenal. My kids, one year, made me past and present pictures in which they took a photo of them as kids and recreated the scene with them as young adults.