your best bet would be to bring a couple rods and nothing over a 6 or 7 wt and do some swinging/dead drifted dries/nymphing for trout. here are the two better streams for it
Skagit river - opens July 1st every year but isnt very good until end of July early August when the water drops (better because you can wade almost everywhere) this is a single hander fishery because 30-40feet is a long cast on this stream, fish to 16-18inches max unless you hunt the Bull trout which can run up to 10lbs respectively. most people fish 4wt or under on this system.
Thompson - swinging/dries/nymphing is all good on this river. bring CLEATS or be prepared to swim as the rocks are very slippery. fish in the 3-5lb range are very common and will give you a great battle on anything 6wt or under.
Vedder - they release steelhead smolts in summer time and they can be great fun on light fly rods and small flies but most people will be using gear rods and trying to target the chinook that are in the system. these chinook can be taken with a swung fly or nymphed fly but is alot tougher than using a gear rod setup.
now unless your here into the fall you wont be able to fish the Coquihalla OR the Thompson for steelhead. Coquihalla doesnt open until September IF it does open. Thompson used to open around mid-october but usually not until Nov 1st and thats also IF it opens (its opened the last two years now)
so as you can see it would be alot better if you planned on doing some trout fishing instead of hoping for summer steelhead. there is summer steelhead in this area but noone gives up that info easily and NEVER online
I would be glad to take you out fishing on the Skagit when your here if you would like someone to show you around a bit. just PM me when youve gotten to BC
good luck !