The SLO Museum of Art occupies a small building at the edge of Mission Plaza, steps from the creek and directly adjacent to the Mission. Admission is free, which removes any reason not to look in. The collection focuses on California artists, particularly those from the Central Coast, and rotates through temporary exhibitions several times a year.
It's a small museum by any standard — you can see everything in an hour — but the quality is consistent and the setting is good. Coming here after the Mission gives you a natural circuit around Mission Plaza without backtracking. The museum shop is genuinely interesting, which is not always the case. If you want to sit afterward, there are benches along San Luis Creek just outside.