Unless you know they collect something and/or really like knicknacks I'd suggest staying away from small items that need dusting. LOL
Consumables are good. Wine, cheese, good bread, chocolates, pates, jams/jellies, salsas, maybe some fancy dessert that's a bit of a splurge that they might not buy for themselves.
If they have kids you might consider a board game, a "night at the movies" pack with bottles of soda, microwave popcorn, candy, certificate to a video rental place, a lawn dart or other outdoor game.