I've only used 4 28mm lenses, all of them being 2.8: Pentax-M, Pentax-A, Rikenon XR and Focal MC (on my Minolta - not a bad lens actually!).
By far my favorite is the Rikenon. But don't take my word for it - there is this lady on Flickr called Georgie_grrl. Check out what she does with a K1000 + Rikenon 28mm f2.8 combination:
Flickr Search: k1000 rikenon 28mm 2.8
There's none on Ebay right now, except for the P version, which doesn't seem as well regarded, at least from the couple of reviews on this site. Also, the P version has the Ricoh pin, which you would have to extract, to be able to use the lens on a Pentax camera.
By the way, I would also take a small sharp 50mm lens with me. You can't go wrong with the Pentax-M 50 1.7 - it's small, almost like a pancake lens, super sharp (very similar design to a Summicron - in fact the back half of the lens is an exact copy of the Summicron). Or, I have a lens that is even smaller and I like just as much: my Ricoh-made Auto Sears 50mm f1.7. I call it the "original plastic fantastic"
When I put that lens on my ZX-M, the camera+lens combination weights next to nothing.