Agreed on all the counts, and I'll add a polarizer filter to the list.
To elaborate a bit more:
-I'm partial to a belt holder for hikes but I use the *much* heavier K-1; a sling strap will work wonders too.
-I have a small, travel-friendly Manfrotto Compact Advanced with three way head that can be had used for as little as 30€/USD and should work very nicely as long as there isn't a lot of wind. There are of course much better, sturdier options if you need such. Like Ismael, I'd strongly advise *not* to get a cheapo no-brand tripod with flimsy legs and plasticky head - those are as good as nothing the moment someone sneezes in your general direction.
-The Pentax O-ME53 magnifier is nice and helps a fair bit with closeups, but it is not a critical purchase if you want to maximize the impact of your money.
-A circular polarizer is one of those filters that really cannot be replicated in post processing, and helps reduce reflections on water or vegetation. I use a Gobe 3Peak CPL and I'm happy with it, but there are many good options around.
-If you are interested in flash use, play around with the built-in unit you already have included. If that's inadequate, a Metz 48, Pentax AF360 or similar works wonders for not a lot of money - they are commonly found lightly used.
Let us know what types of photography you're most interested in, there is niche equipment for everything