Well you need to save the expensive gift for the wedding, so you can purchase either a gag gift or a symbolic gift. You can get them tickets to a play or a game so that they can spend time together. Or a gift card to a restaurant.
While engagement gifts are not required, they are a nice gesture. The gift could be something for their future home together, something to help them celebrate now (like champagne), something to help in the wedding planning, or just something they'll enjoy. Some couples start their gift registries very early, so you might pick an inexpensive gift from it.