From everything I've heard and read, it's a completely different city. The boundary for what's "safe" keeps pushing east. I've been here a year and a half and I've never lived anywhere where the fault lines of gentrification were so visually obvious (like cranes basically encircling public housing projects).
Besides the obvious suspects of the Mall and museums (which are definitely worth doing), nothing really jumps to mind as "must-do". ESL is a lot of fun, which along with Saufhaus next door and Big Hunt down the street (dive bar) are probably the only decent bars around DuPont. It's a nice neighborhood but attracts a lot of suburban bros on the weekend. I always stop in at Kramerbooks around there, a book store with a bar and cafe attached (open til 1am and 3am on the weekends).
- Renwick Gallery is pretty cool although every single person will be trying to get a new profile picture or insta pic
- Air and Space is probably the most popular Smithsonian museum. I like the Hirshhorn a lot and it doesn't take very long.
- Some of the posts from 2012 mention Toki Underground being in a sketchy area...the neighborhood doesn't look like much but there are some fun spots and it's definitely getting better. Toki's good ramen (depending on where you're from), Maketto is a coffee shop/restaurant/clothing store basically across the street, I'm partial to Little Miss Whiskey's which is a dive-ish New Orleans themed bar. There's a beer garden around there I've heard good things about.
- Good music venues: U Street Music Hall, Black Cat, 9:30 Club, DC9, Rock and Roll Hotel (also on H St and home to a shitshow dancefloor on the weekends), Flash for Euro DJs
- U St and 14th St (they intersect) are kind of the go-to nightlife areas which apparently would have been unimaginable the last time you were here
- Other decent bars around the U St/Shaw area: American Ice Company, All Souls, Satellite Room
I realize that's like all nightlife or museums. I don't eat out that often so I don't have much in the way of food recommendations, though I've heard good things about
- Rasika is a fine-dining Indian place in Chinatown that's basically an institution
- Rose's Luxury in Eastern Market area is supposedly really good but they don't take reservations so you have to wait in line at like 4pm
- Masseria is a new prix-fixe Italian place around Union Market (which is nice enough in its own right), kind of tucked away next to a bunch of African and Asian wholesaler places
- Fiola Mare is seafood in Georgetown
- Thai Xing is a BYOB Thai place that's kind of pricy in an old rowhouse
- Plugging Zenebech near Shaw/U St because it's an awesome hole in the wall Ethiopian place in a city full of Ethiopians.