I think we'll definitely go to Mamquam Lake later in the season, how are the fish there?
Many years ago Mamquam Lake was legendary for it's good fish. First time I was there I couldn't find fish initially and eventually found them holding in holes scattered around the shallows at the south end. Fat 15" fish. The last time I was there, already some years ago, the lake seemed to be overstocked with tiny fish. Alpine lakes with good spawning and poor food tend to progress in the direction of starving populations of numerous miniature fish but you never know. Three times I have gone to alpine lakes known for small fish and found good fishing for quite respectable fish. I never found out why that happened. Had there been a winter kill that only left a small population to feast on the available food or was there always good fish in the lake and the particular conditions prompted them to make an appearance?
Mamquam is best accessed via the Diamond Head trails. It is 15 miles to the lake and it is a three day trip. You can go all the way in in one day, camp at the lake and fish a day and come out the third day. Advantage is you get an evening at the lake. Disadvantage is you have to carry all your gear all the way. The other approach is to camp at the Diamond Head campsite half way and day trip into the lake. You don't get the evening at the lake but you can leave your heavy gear at your camp.
Beyond Diamond Head there are two streams to cross. The first is Ring Creek, a brawling silt laden ugly stream. Parks put a bridge across but winters have taken it out in the past. I suggest checking the status with Parks. I have waded Ring Creek several times but it is a dicey process when the water is high. Beyond Ring Creek you pass the foot of the glacier and there is a 100 yard broad stream of meltwater coming out from under the glacier. On a cool morning you can usually step from rock to rock and get across dry. Coming back on a hot afternoon the water is usually up so you have to wade so use boots that can tolerate being wet. Nicely padded modern hiking boots can go sour if you get them really wet.
Take bug spray. They can be a problem at the lake.
Some other possibilities are: People bushwhack into the south end of Garibaldi Lake from somewhere on Brohm Ridge. I have never done it so can't provide information but there seems to be a known route. Deeks Lake near Britannia Beach has some fish and a challenging trail. Access around the lake is not great but there are two smaller lakes beyond Deeks that also have fish and better lakeside access. Norton Lake at the head of Indian Arm used to be reached via the road from Squamish. I have never been there. Last I heard that road was closed to vehicles but you should still be able to walk it.
Also do you know any good river camping spots during the salmon run?
Wish I did.
Good luck and be careful, there are lots of ways to get into trouble in our mountains.