One of our best trips ever was Cartegena Colombia. It's a very romantic spot and has it all, the evening walks on the city walls are the thing for lovers to do, food scene out of this world, you can get lost in the alleys like Venice, the city beaches kind of suck (so stay in the walled downtown area) but Isla Rosario is not far away and you can hire a private boat to go to some of the most amazing beaches you will ever see and be some of the only people on them. Our Airbnb came with a butler and a cook ( no joke). Super cheap destination and as fun/amazing as any European city.
Currently I am in San Miguel de Allende. No beach here but it is a fun and romantic spot also. Very old world feel and the food scene is one of the best I have seen, every meal seems better than the last. Super romantic and cheap. Lots of expats, you could live here or spend 4 or 5 days. The city of rooftop bars and restaurants and cobblestone streets.
Also recommend Mexico City, it's a world class destination. Feels like Rome with it's size, food, ruins, culture, museums and endless things to do. I think they said only Paris has more museums. You can get a hotel overlooking the main square for very cheap also. You will never run out of things to do in that city. No beach either tho, but do what we did and spend a week there then catch a cheap flight to a small beach town for a few days like Puerto Escondido.