How about just a nice bottle of wine, only if they drink?
Or bake a cake just to show them that you are tight with Martha.:biggrin:
+1.
Doesn't have to be a bottle of wine, but definitely a "hostess gift". Much the same as when you are invited to someone's house for dinner you bring a hostess gift. It is a good way to show your appreciation for being invited to celebrate with them (dinner, holiday, etc) but doesn't look like you are trying too hard and won't break the bank.