The Pentax 31mm
is a stellar lens (best autofocus lens ever for any camera according to some, or at the very least, in the top three), but man, it wasn't cheap. But I decided that I had to have it and that I'd better get it before the rebate ended, so I did.

The 77mm is supposed to be similarly great and a good bit cheaper, I may have to grab one of those myself some day!
I do like the Pentax 10-17mm fisheye a lot, though not everyone likes the fisheye look.
mike_r is right, a good flash is really nice, too. I have the Pentax AF 360 FGZ, there is also a larger and more powerful 540. It really makes a huge difference, enough where it's not so awful using the flash for indoor photos at home; when bounced off the ceiling, you get a pretty natural-looking light.
ukcatfan is also right - the K20D is a pretty amazing upgrade.
For a faster kit lens replacement, the Tamron 28-75mm F2.8 is a pretty common recommendation and I like mine pretty well and it's not too terribly expensive (usually slightly over $300). The Pentax 16-50mm F2.8 gives you a wider shot but at a big jump in price ($700 or so) - it also has ultrasonic focusing and weathersealing but neither are particularly useful on the K100D, which isn't weathersealed and will use the old screw-drive to focus that lens. Tamron is about to release a 17-50mm F2.8 that will be closer to the price of their 28-75mm and probably will perform very well. And there's still the Pentax 16-45mm F4.0, which is always well-reviewed but not particularly cheap and a little on the slow side, but it does go pretty wide.
Tamron and Sigma are also both about to release new 70-200mm F2.8 lenses if you want something longer. Assuming that Tamron is able to match their IQ of their shorter zooms, I suspect than a Tamron 70-200 F2.8 is in my future (and I will be able to get rid of two lenses to help afford it!)