Union Market is one of my favorite spots in the city. It has about 40 vendors, including TaKorean, Rappahannock Oyster Bar, Buffalo & Burgen (a Brooklyn-diner inspired spot), Craft Kombucha, and more. There are both takeout and indoor dining options, and plenty of seating outside as well on a nice day. The only actual restaurant in the market is Bidwell Restaurant, which is more high-end and has its own seating.
Before or after my meal, I also love to poke my head into Salt & Sundry. This store sells tableware, home furnishings, and other various items. It is a great spot for gift inspiration and home decor ideas.
Next to Union Market is also Cotton and Reed (see this article post to learn about this rum distillery) and a pop-up movie theater called the Angelika Pop Up which offers cheap matinee tickets and craft beer and wine.
I recommend spending an afternoon at Union Market, and trying snacks from several of the vendors. Do note though that weekends can get very busy so if you want a seat you should expect a wait.