Like most here, I'd suggest something a bit longer, too...
What that is can differ, depending on your preferences... there are quite a few options...
The DFA 70-210 f4 is an obvious choice, though far from inexpensive, considering its range.
There are a number of 70-210/70-300 lenses available used as well.
I have the old F70-210 f4-5.6, and it is great for the price (they are extremely inexpensive, generally), though not up to the level of the newest lenses.
I also have a DA-L 55-300, which vignettes less than I expected on the K1. It is also less expensive than the modern (and highly regarded) 55-300 PLM, which vignettes a bit more, so I've heard.
Or even recent models from Tamron and Sigma. Inexpensive, but less awesome on a K1...
All of these options are small and light, though you're giving up some qualities for that...
I rarely take my long primes on road trips -- generally just the zooms, because of the convenience.
But my road trips rarely have opportunities for tripods and such, so that plays into the choices...
My 300mm f4 is also an antique, so it's not awesome on the K1. I might change my mind with newer glass
-Eric