I posted a little while ago about the fantastic summer workshops offered by the Fine Arts Work Center in Provincetown, Mass. Another program I've attended more than once, and have enjoyed a great deal, is the Iowa Summer Writing Festival in the infamous Iowa City. Unlike the MFA program at the U. of Iowa, the workshops at the Summer Writing Festival are not competitive to get in -- they're open to anyone. They don't have the rockstar big-name faculty that Provincetown does, but the teachers I've worked with there -- Meredith Stricker, Michael Carey, Kathleen Peirce -- have all been very good. They offer a number of workshops suitable for beginning to intermediate writers (as well as some more advanced possibilities), and in addition to the usual week-long workshop they have a bunch of weekend ones, which are fun. Plus, you can visit Prairie Lights, one of my favorite bookstores!
Full disclosure: I haven't been to the Summer Writing Festival, or to Iowa City at all, in quite a few years. I think the last time I went was around 1992 or 1993. But I'm guessing that it is still a worthwhile program.