Thanks Aubrey and Ckorik. Having spent most of my life near sea level, I'm just no expert on mountainous terrain. I have some general ideas, but I was trying to decide how to handle it.
For the sake of ease, I've decided that the first "random" encounter will use a pre-printed forest map that I have, and I'll just say that there are some trees between the mountains that they're passing through. That will be when they're first entering the mountains.
I'm thinking for the giant patrol close to Jorgenfist, I'll draw a trail along the side of a mountain, with a cliff wall going up on one side, and a cliff dropping off the other. Then we'll go for some bull rush tactics and have fun with it. They should still win pretty easily, but someone falling 100 feet or something could slow them down, damage them pretty badly, and maybe give the giants time to run back and alert the fort of pending intruders.
In re-reading the next section of the adventure, I realized my PCs may screw things up some. Conna's supposed to be waiting for them, based on knowing that the giants were defeated in Sandpoint, and expecting the adventurers who did it to come to Jorgenfist. But my party intentionally used teleportation to try and get to Jorgenfist before word of the battle in Sandpoint could get back there. Will Conna still have any reason to expect them?
In my case, my group actually let Barl Breakbones live after beating him and his troops at Hook Mountain, so that probably solves the dilemma of how Conna would know of the PCs. I'm debating if Barl should be back at Jorgenfist, happily anticipating round 2 vs the PCs, or if Mokmurian would have killed him for his failure. I like the idea of the group getting to fight the same bad guy a second time, this time in a tougher environment, with more giants around. But it does seem like M's style to kill those who fail him.