Yeah, give them a simple task. Something that any reasonable programmer should be able to do in 15 minutes. Then give them a solid hour to work on it. If they can't produce something working in that time, that's a pretty informative result. The time limit isn't a speed challenge; it's meant to be very generous, and act as, "Look, we need to move on..."
Personally, I like to give a few different options from which they can choose freely. Something procedural, something OO, and maybe something in SQL or a functional language. Perhaps a couple different choices for each - around 6 to 8 total. That way you don't run the risk of excluding a worthy candidate because you happened to design some problem they aren't really specialized in, and if they can't handle any of them, that's a nice big red flag.