For our parents big anniversary we threw them a “prom”. We used my house because I had a big connected living room and dining room. We cleared it all out and put up basketball hoops, taped the floor to look like a gym floor, rented a disco ball ;) and had balloons set up to drop from the ceiling. We played the old songs they loved from their high school days and we all dressed up like it was a real prom. We had a fancy car pick them up at their house and bring them to mine where we were all waiting. We had punch and cake available and we made crowns and sceptors and made them king and queen of the prom :D They LOVED it. Oh we also put black plastic under the curtains so they couldn’t see in til they came inside and it made it real romantic with the lights off and cheesy disco ball on ;)
For another anniversary we hired a painter to paint a famous pic my mom loves of a boathouse party (i think, I forget the painting’s name) but replace all the people in the pic with paintings of all of us in the same poses! and our wives and kids. They LOOOOOOOVED that! We all got together in secret in clothing that was “close” to what they wore in the painting and we all posed and the painter took photos to work off of. It turned out really neat. :D