April is right at the end of the rainy season in San Francisco. The weather migh be lovely or might be rainy- but, hey- that's what it's like here. If you wait til May, you're pretty well assured to have really nice weather- or even later in April- but chilly either way- especially in the evening.

As for Yosemite, depends what you want to do. You've had lots of other responses about skiing, etc. But that may or may not be what you're interested in.

You may be interested in staying in the lodge or at the Ahwahnee- or even a tent cabin- and going for walks in the snow, building a snowman, reading a good book while looking out at the snowy mountains and fields, etc. I've done that in November and April and found Yosemite lovely. Maybe there's even a hill (though I can't remember where) to go sledding, etc. If there's been any warming at all, the falls will all be flowing beautifully and walks in the snow in the valley (or even driving) to the bases of the falls might be beautiful- with varying degrees of strenuousness. Just bring very warm clothes. (I moved to San Francisco in late May, 1968 and the very first thing I did was to go out and buy a warm pea coat- and wore it all summer.)

There are also painting and photography classes- though I'm not sure they have them other than the summer time. You could call or e-mail them to find out. I'm also not sure if they have the evening ranger programs in April or May either.

So I think the answer to your question is- depends what you're looking for. My feeling is that San Francisco and Yosemite are two of the most beautiful places in the world and the weather is merely an inconvenience at times!