To get students to buy-into using their own time for art enrichment will require a little creativity. In my case I offer extra credit for those who go. (10 points added to any test, project, assignment, but not benchmarks) I do teach in a city that happens to have a lot of galleries and art events on a monthly basis, so I could make it mandatory that my advanced students visit one art event per semester and bring documentation of their visit. If they take photos and create a Google slide show, it could even become a "show-and-tell" enrichment moment to open or close a class.
I have created 2 forms you are welcome to download and use for either scenario: Museum Visit or Gallery/Art Event. Both require students to do some writing and reflection as documentation of their participation. When they use a worksheet, I find it really helps them to focus.