Mt. Pleasant Farmers' Market as a mid-sized market and is packed with produce stands. Regrettably, this market happens at the same exact time as the Mt. Pleasant market, on Saturdays from 9am to 1pm. The U Street market is only scheduled to last until November 17th this year. So Mt. Pleasant has them beat on that front, as they are scheduled through December 22nd.
Bloomingdale Farmers' Market, another neighborhood market that I've frequented in the past. Both markets emphasize local, seasonal produce. You won't find any lemons that fell off the back of a truck. (I'm looking at you, Eastern Market.) The 14th and U market also has a mushroom stand.
For my market haul, I scored fresh, local greens (kale, arugula, and a mustard greens mix), garlic, onions, pears, apples, salad tomatoes, grapes, green peppers, and hot peppers. I haven't bought any winter squash or pumpkins yet this year but they are omnipresent. Maybe next week I'll finally take the plunge.