Rainbow trout stocking is scheduled to begin next week in the Lower Mainland. These fish might not fight as well as wild trout in rivers, but the frequency of bites can keep the day interesting while you practice flycasting. Keep an eye on this thread as I will post up stocking updates once lakes are stocked.
http://www.fishingwithrod.com/yabbse/index.php?topic=20043.0Some lakes to try out include:
Sasamat Lake, the floating bridge is a good place for flycasting and catching fish can be easy if they are surfacing nearby.
Lafarge Lake, the beaches are good spots for flycasting, waders are necessary if you want more backcasting room.
Flycasting from shore is doable at the other lakes too, but backcasting space is limited. A float tube or pontoon boat would be advantageous if you have one.
If stocked lake trout fishing is not preferred, then cutthroat trout fishing would be the best alternative. Stave River should be fishing very well either now or in a few weeks. Both cutthroat trout and mountain whitefish should be plentiful on the surface. Casting space is also not too limited, especially on the east side of the river below the dam.
Fraser River between Chilliwack and Hope is another place to focus on. Spots that have easy access are pretty quiet this time of the year, very enjoyable. Cutthroat trout fishing is hit and miss, but when you hit them, it's often a pretty good day. On miss days, you'd just be practicing on your flycast.