Give them a memento that celebrates their unity. It will be an added bonus if they will be able to use your gift during their wedding! For example, crafty people can create an innovative guest book for the couple. For closer goofier friends, perhaps you can consider buying sex games for the couple. ;)
I think the best gifts for a couple are the ones that help them bond closer or spend some quality time in a meaningful way. Some ideas that come to mind are cooking lessons, pottery lessons are even art jamming. The activity really depends on what type of couple they are. Material items that encourage doing things together are great as well. Adult-oriented board games might be worth a try...
The best engagement gift for a couple is a gift that is given with thought and sincerity. While store bought gifts are okay, the handmade options tend to be the best. At a minimum, all gifts should come accompanied with a handwritten note that demonstrates intimacy and support. Coupling gifts with the couple’s interests and motivations is always advisable.