I asked myself this question recently, what would be a good setup to take on a casual biking/hiking trip.
One of the strongest options I considered was to just add a prime (a DA 21mm) to my K-S1 or K-50 and be done with it.
A 2nd option I considered was a cheaper RX100 or one of the equivalent Canon cameras, if I could find it. Of course their advantage is the pocketability. The only one that I *might* be able to consider price-wise would be the RX100 mark I or II with its f/1.8-4.9 lens. That lens wouldn't be much better than a f/3.5-5.6 standard zoom when you consider aperture, and it would be quite worse at the long end. But portability is good, except that it also means cramped controls, so I gave up on that option rather quickly - there are DSLR setups that aren't very heavy so I decided to go just for that - and my decision came down to two options, a DA 21mm or a light zoom (so something like the DA 16-85mm, DA 18-135mm or DA 18-250/270mm was out, too heavy).
The option I ended up choosing was the light, retractable HD DA 18-50mm f/4-5.6 lens on my K-S1 or K-50, and I was able to find a used copy for a good price. The range isn't that different from an RX100 (28-75mm instead of 28-100mm in the case of the RX100 mk I and II, 24-70mm in the case of Mk III and newer). The images are sharp from wide open and have beautiful color and detail (the HD version of the 18-50 design, I feel, finally gets the best of the original 18-55 design, which I suspect was under-coated, and the HD DA I suspect has coatings in more elements, hence the "higher resolution" claims in the Ricoh website).
With the K-S1 the setup looks like a bridge camera - I was able to sneak this into a wedding a couple of weeks ago and nobody cared. I wasn't going to take pictures of the wedding itself, I just wanted to take some pictures of our family in that beautiful farm setting, while we were all dressed up
But with the K-S1 there's an issue with shutter shake, so I might take the lens on a K-50 sometimes instead - not quite as light but still quite lighter than the K-3, K200D or K10D. Or I'll just take the K-S1 anyway and watch my shutter speed.
I'm quite happy with this purchase... in case you're interested in the thread where I discussed this, it's here:
Crazy?? DA 21 (SMC) vs HD 18-50mm for very small walk-around lens - PentaxForums.com