golf (they both love), wine (she loves) and Miller Lite (he loves).
This will be an 8" round cake since it is only for 15-20 people.
You could make it look like the top tier of a wedding cake be makeing it a gorgeous all white tier and then on the bottom add grass and a few golf balls. At the top instead of a bride and groom you could put a mini wine bottle and mini miller lite beer can.
Good luck. That is a hard one.