MOMA Wales. The Museum of Modern Art is in Machynlleth in mid-Wales, attached to Y Tabernacl which was a church, but has been converted to a space for the performing arts. At the back an old tannery has been restored and converted to add additional space and interest.
The museum is small, but has several exhibition spaces, and features the work of Welsh artists. They have a permanent collection but they also change the exhibitions often to include other works of interest. Every summer there's an art competition - this year's theme was Coleridge's poem "Frost at Midnight".
I really like the old buildings and although it lacks the glamor of say, MoMA New York (which I visited in May) it's smallness means you can visit without being overwhelmed. And entry is free.