While the museum could be better, this was still an interesting stop to see the statue, just think about the snow being as high as the base of the statue. Donner Lake was pretty. Also make sure you do the short walk into the woods to see another cabin site from the Donner Party, which used a large boulder (still there and marked with a plaque) as one the walls/fireplace. If you're not sure where to walk, just ask at the museum. There is another site where the actual Donner family stayed, but I didn't stop there and I forgot to ask about it's location at the museum. I'm not sure what's there. It was interesting drive in on I-80, thinking of pioneers trying to cross that summit.
As far as parking goes, there were plenty of spots in the afternoon. I could see how the middle of the day or on the weekend might be a problem parking, but I think early or late is a good rule. If you can't time your visit first thing in the morning, maybe you should aim for later in the day. We stopped at around 4pm, and the parking lot was almost empty and the woods and lake were pretty quiet.
If you own or manage Donner Memorial State Park and Emigrant Trail Museum, register now for free tools to enhance your listing, attract new reviews, and respond to reviewers.