Congrat for the K-5.
If you have already the 35 f2.4 prime, I am not sure why you want another lens around 30-40,
I would recommend a 50-55mm with large aperture for portrait and low light. There are many choices incl. Pentax, Sigma, Voigtlander and Zeiss. The Zeiss and Voigtlander are MF and highly regarded. (I love my Voigtlander Nokton 58mm and I would recommend it highly.) For AF, Pentax and Sigma have some. You may look at the Lens review in this forum and the independent lens reviews at
www.photozone.de.
Another suggestion is some macro lens around 100mm. There are 3 great lenses: Pentax 100mm, Sigma 105mm and Tamron 90mm. You would not be wrong with any of these 3.
A further suggestion is to consider the Pentax pancakes: they are really small lenses and this is an advantage (in my opinion) when you travel.
Hope that the comment will help...