I've always thought that something like a 40-120mm f2.8 zoom would be the ideal walkabout lens. I always find 80-200mm (ish) lenses too long, but a 35-70mm isn't quite long enough so you end up lugging two about (or more likely going back to a 28/50/120mm triple as the size and weight is about the same).
Pentax made a K mount 45-125mm but only for three years. It's supposed to be a decent performer and I do keep my eyes open for one at a sane price, it's a bit slow at f4 but that range would cover a lot of shooting.
They did!? I want one!! (well, maybe I don't: I do have enough trouble carrying 3 primes about, and so we turn full circle and address the issue of prime weight vs zoom weight... )