Plenty of option during that time for what you want to do. Kawkawa Lake is hit and miss in April. It can be really good, but it can also be just terrible. I don't really recommend it if your fishing days are limited during your visit.
Whistler area lakes should be ice-free by then since it has been pretty mild, but I'd keep following updates and see how the melt is. Alta Lake is a good catch and release lake to hit for both rainbow and cutthroat trout.
Cultus Lake can be good during that time for cutthroat trout, kokanee, resident coho salmon, northern pikeminnow, largescale sucker. Focus on the creek mouths.
Hicks Lake just north of Harrison Hot Springs also has a variety of species. They include stocked catchable Fraser Valley rainbow trout, native rainbow trout, cutthroat trout, kokanee.
Weaver Lake has tons of rainbow trout but they are mostly on the small side.
Jones Lake has cutthroat trout, rainbow trout, kokanee. They are aggressive, good size, but the drive up there can be a bit rough, need a good 4WD for it.