I haven't been to Lake Madrid, but I love Bass lake! You should really check it out, it's one of my favorite places in the Bay Area. You have to hike in 3 miles to get there. Go to Stinson, turn off towards Bolinas on the road into town and turn right when you see the sign for Point Reyes Bird Observatory (before you hit the main drag in Bolinas). Drive to the end of the road, a few miles, it turns into a dirt road, and there's a trailhead to Bass Lake. It's a beautiful secluded peaceful lake with a great rope swing. There is a field nearby where you can sunbathe, but there is no beach. This time of year it might be foggy there...I don't think you can camp there but it's definitely an easy day trip from Berkeley.
Other favorite lakes are up in the Tahoe area, which about 4 hours away. Lake Tahoe is of course stunning...but if you want a more peaceful place away from the crowds, try Fallen Leaf lake, which is an exquisite green lake just south of Tahoe (Go to South Lake Tahoe and take 89 towards Emerald Bay. Stop at the ranger station and they can tell you how to get to Fallen Leaf. It's just about 10 min from South Lake Tahoe. There are dozens of other crystal alpine lakes up there, so you might want to hike into Desolation Wilderness (you don't need a permit if it's a day hike) and jump into some of those lakes (freezing, but refreshing).
I'd love to hear more info about Madrid Lake--does it have a beach where you can lounge? Do you hike in or drive in? is it crowded?
Thanks!