Yosemite Valley view in the Winter by Ian Carvell
If you do nothing else you must explore Yoesmite valley. You can’t really avoid the crowds here but you should see it. There are valley loops that are good walks or cycles. I personally enjoy walking through Cooks Meadow, to Lower Yosemite Fall and to Bridalveil Fall. Taking a dip in the Merced River in the valley is also lovely. I’d skip the museums in the valley unless the weather is bad as the crowds are crazy and it’s better to be outside in the park anyway.
Yosemite Valley from Half Dome
If you fancy something a bit more challenging try the Vernal Fall trail. It gets popular too but the falls are stunning. It’s an uphill hike but the views are totally worth it. Once you get to the top it’s not too much farther to Nevada Falls if you’re feeling strong. Please be careful by the falls and do not enter the water. At the top you can either retrace your steps via Vernal on The Mist Trail (hopefully you’ll know why it’s called Mist by then!) or take The John Muir Trail down.
For a less strenuous hike on the flat try Mirror Lake which usually has a lovely reflection of the granite around it floating about on top. Very picturesque.
If you have time explore other parts of the park such as Tuolumme Meadow.