As with any activity the more experience you get, the more refined your approach becomes and so it has been with me. My first activation was last March. I took my KX3 with an FT-817 as backup, an Alexloop, a Buddistick, several coax jumpers (BNC - PL-259, BNC - BNC and other combinations just in case), a pound of trail mix and several bottles of water. All this for a 3.5 mile round trip. I didn't even want to weigh it.
I started with the black Kelty, then the red North Face
and finally I've progressed to the CamelBack Fourteener in the middle
I started with a big pack and I filled it up, which is a big mistake. So I realized maybe I should downsize a little. So I began to rationalize what I carried. I realized that the KX3, while a very nice radio, is a lot to carry in both weight and bulk. I am really more of a CW guy, so I was carrying around a lot of capability that I didn't need. I owned an ATS-4 which weighs a few ounces and is a CW only rig with 5 bands. Also, the Alexloop is an effective antenna, but it's bulky and takes a lot space in a pack. For relatively long hikes, it's too much. The problem with bulk is that it makes you get a bigger pack and human nature dictates that you fill it up, so you unconsciously take more stuff. For an antenna I went to a trail friendly EFHW 40/20/10 supported by a carbon fiber fishing pole that telescopes to 21 feet.
If I am on a drive-up/walk-up summit, I might carry some of the bulky stuff because it's easy. I recently used the Alexloop on Mt. Locke because I didn't have to carry if far, sot the situation can dictate a different configuration.
The other consideration is water. You should get a pack that allows you to insert a water bladder with a drinking tube accessible to you while you are hiking. This not only allows you to drink on the go, it is a better way to carry water and eliminates the need for bulky water bottles.
As you can see above I have continually downsized my pack. The CamelBack Fourteener is about 1/3 the capacity of the Kelty that I started with. Below is what I carry.
Typical Activation Load
My radio is in an iPad portfolio along with the power supply, paddle and ear phones. I carry two antennas, a modified Buddi-stick and a carbon fiber telescoping fishing pole with an EFHW antenna, VHF radio, coax, GPS, log, first aid kit and rain gear. The pack above weighs about 9 lb. The bladder in this pack holds 100 ounces of water (gallon ~ 128 ounces). I been on an activation in the heat that was 9.5 miles round-trip and I had water left over, so you don't have to fill it up every time. Without water and without any food/snacks the above weighs, including the pack, about 9 lbs. I do have a redundancy with the antennas, but that is to allow for different conditions and what I have time for on the summit. If I lived in the mountains, I would only take one antenna.
I have climbed Mt Sherman, 14,036 feet and Emory Peak, 9.5 mile round trip using this pack and I haven't lacked anything that I needed.
However the game continues, I am still looking for ways to cut back.