Hello,
i had to face a similar question when i moved for 14 months to Japan and wanted a light weight and small travel kit. My focus was on hiking, towns, sightseeing and so on, but not on wildlife photography (didn't want to disturb the bears
). And i am a sucker for primes
Ok, i chose the DA 15, DA 21, FA 35 and DA 70. For longer tele, i had the TC Pentax-F 1.7.
When i went hiking, i took the 15/35/70 combo and use the 15 most of the time. For towns and temples and so on, it was a tie between the 15 and 21. All in all i have to say, that i used the 70 far less than the other lenses, but i think this is mostly due to my preference.
In retrospective it was a very good choice, but i could have lived without the DA 21
The kit was light enough to be used on 10h hikes in the mountains and not being a burden. Most of the time i changed between the 15 and 35, so i put the lens i didn't want to use in my pocket - very handy
I exchanged the caps of the primes with snap-on caps like the FA 35 has, especially the cap of the DA 15 is very annoying. In the case of the DA 21 and 70 it is not so important to exchange the caps, but you should do it when you share one filter for every lens. Sharing the filter is nice, but i will buy one CLP for every lens, as it is sometime a little hassle to change the filters.
So from my experience i would always recommend the 15/35/70 -combo. You can take great pictures and you will always have a smile on your face when you see a Nikon or Canon guy dragging the 2-3 kg of his precious camera and fancy 2.8-zoom dangling from his neck
Did i miss something ? In some occasions i missed a longer tele and i am seriously thinking about the 50-135/2.8 - which is not so long, but i think the longer tele are just to heavy and thus not usable for longer excursions.
I hope i could help you in your decisions.
Tobi
p.s. just a random pick of a picture by the DA15