Recently my dad was telling me about how when he travels, he finds himself driving around in neighborhoods with his laptop on the passenger seat to help him locate open WiFi networks. So when I was at Staples the other night I bought him a Kensington WiFi Finder Plus:
It seemed to work pretty well detecting WiFi networks, but it wasn't able to detect my bluetooth phone. Since he is using it only for WiFi this won't be a problem. The gadgeteer has a full review on it here and runs into the same issue.