Located in the heart of a cool part of town, you will be walking distance (or a short uber) from some of the best spots: Contramar (seafood dinner, closes at 9pm), El Palenquito (Mezcaleria - try it with Jamaica con hierba Buena), Gin Gin (bar or come earlier for dinner), Yuban (cute restaurant). For clubs, try El Paraiso or El Departemento (house music) but beware showing up late and getting stuck in the massive crowd vying for space.
As for the apartment itself, it is great to have 24/7 security guards (tip not required when they let you in). The apartment has two big queen beds and (bonus!) two full bathrooms. Perfect for a group of friends or two couples. Fully equipped kitchen. Simple interior (but don't choose to spend time inside -- choose if you're gonna be out exploring all the time -- it is small and simple).
Guillermo was super communicative and really nice.