Most places are just casual in the area. But Im not as familiar with the Green Valley resort, someone else may post with more info, but I doubt anyone is going to be familiar with it.
But lots of people are usually hiking ect in the area and not too fussed about what you wear from what I have seen, so I would not worry about that.
You may find you get a good rate since you are staying at the Spa on Golfing in the area. I would check with them first. There may be some who are familar with golfing rates in the area that may post as well.
I have heard that the food is good and doubtful you will want something else, but there are places to pick up snacks, ect not too far away from what I remember, a Green Valley Market just about a mile away or so in that area.
As far as swimming in April, that would just be a hit or miss type of thing. Some years it may be nice enough, it really just depends.
Edited: 10:34 pm, November 08, 2013