I have been to four of the national wildlife refuges now and it makes me appreciate the San Luis one even more. Just for starters, it's free, they have a nice visitor center and you can get out of your car wherever you want (but you will spook the birds more easily if you leave your car). As far as the attractions go, they have a tule elk driving loop, a wetlands driving loop, a spur road with fishing spots and several places to pull over and take a hike. So, you can spend most of the day there if you really want to slow down and investigate everything.
Of course, you'll see the most wildlife if you go in the wet season. I guarantee you'll see a large amount of waterfowl and raptors. I went in August the first time and most of the wetlands were dried up and the temperatures were in the upper 90's, so I don't recommend that. But I would recommend picking up lunch in Los Banos and heading out to take in the wildlife and scenery at San Luis. If you have the time, the Merced refuge has lots of wildlife and a nice auto tour.
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