New to amateur radio, or a seasoned operator some people have never experienced the fun of operating out portable. The thrill of operating in a low noise environment.
VHF/UHF
If you going to take out your VHF/UHF handy, make sure you have a good antenna fitted. The stock antenna or the fake cheapo antenna on eBay is exactly that mostly useless. VHF operators sometimes take a vertical antenna with some gain to take advantage of an elevated location.
If your hill walking with unlicensed operators, then consider some PMR 446 radios as they provide good range and the ability to keep in touch with your fellow walkers.
HF Antennas
Speaker wire makes excellent antenna wire, lightweight and strong, and I would suggest you make a few dipoles and maybe wind them onto a former so they are easy to deploy.
If you are taking the equipment in the car, then you may want to consider getting a telescopic mast. These can be deployed behind the car with a suitable mount.
Para style cord is great for putting up guy ropes. You will also need some tent pegs to secure items in the ground. Flimsy tent pegs often bend, why not try these more robust pegs.
Guy line tensioner will come in very handy when putting up a mast.
Telescopic plastic poles used to fly flags at campsites make ideal HF antenna supports.
Power
No one wants to run out of power for their phone while out on the hills. These USB power banks will recharge your phone making sure its ready when you need it.
Everyone needs a torch, this one fits on your key ring and provides best in class illumination.
Depending on power requirements, you may wish to use a leisure battery or a suitable voltage LiPo system. If your going QRO then a small generator will provide the power needed.