D90 lense choices (for an old fart)

I think that a wide angle zoom (10-20, or instance), might just be the thing. There is big difference between that and your 18mm wide range. It will give you a whole new perspective, so to speak, but does require modifying your technique to get full advantage (and it sounds as though you are looking to learn something new anyway).

If you might like to expand into the Macro area, consider getting a set of Kenko extension tubes (which are quite cheap) which would allow you to use your existing lenses for macro-like work. Or, pick up a NIkon 60mm macro lens.

If candid, low light interests you, consider the Nikon 35mm f1.8 - also quite inexpensive and good quality.