Most museums allow sketching with dry media (except pastels) without special permission, and ban liquid materials like ink and watercolor. I use water-soluble graphite pencils, a sink in the museum’s Men’s Room and a brush to blend some of the sketches, sometimes with unexpected results and quizzical looks from other museum visitors.
I find sketching meditative, it slows me down and I connect with specific art pieces. The point is not an accurate replication of the original art, but rather to spend some time focusing on a specific piece of art.