Any ideas of where to take someone with dementia that is not too crowded?

Not being familiar with your region my ideas may or may not be appropriate. But here are some thoughts. We have a County library that has a small area dedicated to local exhibits -- artists, collections of shopping bages, you name it. The library is quiet and peaceful during the daytime. Even listening in at the children's story telling session in the library may appeal to her. Ditto small museums or historic buildings. Some of our local churches have afternoon music sessions, one here has it at noon on Fridays and others have music on weekend afternoons ... especially at this time of year. If she was a shopper, a visit to some churches holiday bazaars or white elephant sales may be of interest. Check for events in your area on websites and in houses of worship. There may be a children's sing a long for example. Music generally sticks with folks as they experience dementia as it is a long term memory not short term. I know my church has a music selection event for brides twice a year. Everyone is welcome to come and the musicians play a selection of wedding appropriate music. It's free, you can sit in an area you can easily depart from and the music is lovely.
Hope this sparks some ideas for you, good luck.

She is 50 years old so a senior center seems inappropriate at this time, and she does not wander off in the winter because she hates the cold (and cannot drive)

Try your nearest Senior Center and sign her up. Fair warning, if she cannot stop wandering there, start looking at memory care facilities. If she wanders off in the winter, she will die in the cold.

I also want to note that I am from Iowa and it is COLD. So something preferably not outside (and if so, not outside for a long time)