For day trips in the Lower Mainland, close to Mission, perhaps
Weaver Lake in Harrison Mills if size of the fish is not a factor. There are plenty of rainbow trout in this lake, stocked Blackwater strain, but their average size is between 10 and 14 inches long. Lots of fun, as we found out last year.
http://www.fishingwithrod.com/video/video_blog/2012/05-09-float-egg-trout.htmlThe gravel road to the lake is a bit steep and can be a bit slick when wet. Camping is available at the lake, there is also a day-use area with a pretty well maintained boat launch.
Kawkawa Lake produces well between now and the end of April. Fishing tapers off in May but picks up again in late June. July and August are not as good due to jet boat traffic. It produces kokanee, cutthroat trout, landlocked coho salmon and the odd rainbow trout.
Cultus Lake should also be considered in the next couple of months. You have the opportunity to troll for cutthroat trout which are preying on salmon fry, or bottom fish with bait for bull trout and a variety of species such as northern pikeminnow and largescale sucker. Largescale sucker school in the shallow at this lake, and can be surprisingly challenging and fun to catch on the fly by stalking them.
A little further than these two lakes is
Alta Lake in Whistler. This catch and release fishery has a high abundance of rainbow trout, which are similar to the ones found in Weaver Lake. Among these rainbow trout, you'll also find the odd triploid cutthroat trout stocked by the Freshwater Fisheries Society of BC. These fish can be as big as 3 or 4lb.
These are not my favourite lakes, but they are the usual lakes I visit with my boat in the Lower Mainland between March and May. Good luck.