I was just there for 9 days in late April.
Planned for rain and wind and got a sun tan instead!
We used this map for planning the trip:
Map of Iceland for photographers, hikers and tourists
I used the MindShift backpack to carry all my gear:
MindShift Gear rotation180° Professional Backpack Deluxe 210
I highly recommend this to hold your gear.
I was able to fit in the waist belt: K3ii with 16-85, 1.4x converter, DA 55-300, DA 12-24, DFA 100mm macro, 15mm and a Samyang 8mm. Tight squeze but it works with APS-C!
I stacked 6 lenses together with a double lens mount cap to save space:
OP/TECH USA Double Lens Mount Cap for Pentax Lenses 1101251 B&H
Also brought a tripod, Nikon SB-30 and monocular.
And bring a hat that ties under your chin!
The K-3ii has GPS built in so bring an O-GPS1 if you bring your K-3.
Lens usage: 16-85 was lens used most often followed by the 55-300 (sometimes with the 1.4). Used the 8mm once and the 12-24 a few times. Never used the 15 or the 100.
I decided that if I needed a wider angle I would just use the 16-85 and take several pictures to stitch later.
Now when I travel I only bring my 16-85, 55-300, 1.4x. flash and filters.
Here's a few days worth of images - haven't processed them all yet:
Iceland 2016
Have a great trip and watch out for sheep!
Chris