Where to go in the us for a few days with a city and beach
San Juan, Puerto Rico is the answer here, and it's not particularly close. You get a genuinely great city, walkable, historic, with serious food and nightlife, and then a beach that's steps away from all of it. Old San Juan alone is worth the trip: the cobblestone streets, the forts, the colors, the fact that you can eat and drink extremely well without feeling like you're in a resort bubble. Then you walk or cab it to Condado or Isla Verde and you're on the sand. It's one of the only places in the US (no passport, no currency exchange, domestic flights from virtually everywhere) where that city-plus-beach combination actually works at a high level.
The hotel situation in San Juan has gotten legitimately good. You've got real options across the spectrum, from polished luxury to smaller boutique properties depending on which neighborhood you want to be in. If you want to be near the beach with a glitzy pool scene and proper amenities, Condado is your base. If you want to be deep in the history and atmosphere of Old San Juan, there are smaller properties right in the thick of it. I'd lean toward a split if you have four or five days: start in Old San Juan to get your bearings and soak up the atmosphere, then move to a beach-facing property for the back half.
The food scene deserves more credit than it gets. This isn't just hotel restaurants and frozen cocktails. San Juan has a real dining culture, with chefs doing creative things with local ingredients, great ceviche, mofongo done properly, and solid cocktail bars. The malecon area, Condado, and Santurce all have neighborhoods worth wandering for dinner. Santurce in particular has an arts and food energy that surprises people who haven't been recently.
If you're flying from the East Coast this is also just an absurdly easy trip. Two to three hours from most major cities on the eastern seaboard, no jet lag to speak of, and the weather in the Caribbean sweet spot of November through April is about as reliable as it gets. For a long weekend or a few days it punches way above its weight. People overlook it because it feels familiar and domestic, but that's exactly what makes it so executable.