Pentax lens selection

I have the 17-70 f4. it is a good lens, which is already sharp fully open on the 24MP K3, and 1 stop brighter at the long end than the 16-85 f3.5-5.6, which is nice in low light or for portrait.

Both lenses are very close in size and weight. Their optical formula also look close, thus I think the newer 16-85 is an evolution of the older 17-70, trading the extended range at both end versus the variable aperture.

Please note that the 17-70 has a rubber gasket around the mount but is not sold as "WR".

Fast f2.8 zooms are big and heavy, which is not the most convenient when traveling.

If you are used to compact cameras or bridges, beware that a DSLR with a 16-85 f3.5-5.6  lens is much bigger and you might not like it.

Also the larger sensor delivers a much better quality, with a shallow depth of field, which is nice for subject isolation but requires a more precise focusing.

For travel, mirrorless M43 ILC cameras might thus be worth a look.

For indoors shooting in museums, you might have a look at a fixed focal Pentax prime as a second lens in your bag, for instance:

• DA 35 f2.4 or DA 50 f 1.8, which are both light and inexpensive, very sharp and resistant to flare. DA 50 f1.8 would also be a good cheap portrait lens, whereas DA 35, with its wider angle close to "standard" in APS-C, could also be more useful in crowded rooms.
• DA 40 XS f2.8, which is so tiny you wont notice it in your bag.

These lenses are not weather resistant, but for indoors it doesnt matter. There are better lenses, like the "limited" primes, but of course more expensive.

As for me, though I have many Pentax lenses (see my gear list), whether primes or zooms, my favorite for travel is DA 18-135 f3.5-5.6.

It is not as sharp at pixel level as DA 16-85 or DA 17-70, but in most scenes you wont notice the difference in printings up to A4 (8"x12") nor in viewing the full image on a HD screen, and even on a 4K 55" OLED TV.

And it is a much more compact rugged lens which allows me to go with a all in one lens (save for wildlife where long telephoto is needed) from 27 mm to 202 mm full frame focal range  equivalent. If you are interested, you can buy it bundled with any new Pentax camera at a very good price.

Also please note that big cameras are not welcome everywhere, and a high end tiny compact with a bright lens and 1" sensor may be a useful complement or alternative when traveling: there are different offerings from Sony, Canon or Panasonic for instance, usually with a 24-70 FF (16-45 APS-C) equivalent focal range, they fit in a pocket and are very light, and their maximum aperture around f2/2.8 on a 1" sensor will give you the same subject separation and low light performance  as f4/5.6 in APS-C.