We usually stay in San Juan either before or after our charters - and sometimes both. As far as beachfront in the San Juan area, Isla Verde has the best beach, but Condado has more to do and is still on a decent beach. Resorts we've stayed at and recommend include the El San Juan, San Juan Marriott, San Juan Ritz and La Casitas (located at the El Conquistador in Fajardo). The El San Juan has always been fun with a lot of action in the lobby, but the last time we stayed there was just after the remodel and it seemed to be missing that vibe. They closed the casino so it could be that without that draw the electricity in the rest of the resort disappeared as well. Or maybe just a fluke. The Ritz is always nice, but it is slightly more removed from the main Isla Vede strip and more low-key. That said, the Isla Verde strip is a bit more utilitarian and doesn't offer the options the Condado or Old San Juan do - but the Isla Verde resorts are nice.
We stayed at the Marriott last trip for our first time and loved it. It's located in the heart of Condado and is a very short taxi ride or a long (but doable) walk to Old San Juan. If you stay in Condado I would recommend taking a taxi to San Felipe and then walk back down through OSJ to Condado - you can always grab a taxi if you get tired. Plenty to do around the hotel as well and though it is on the beach, if you swim in the ocean you have to be a bit careful of the currents there. They also generally have good pricing on their rooms. We stayed in a corner suite with great ocean views for barely more than a standard city view room at the other resorts.
La Casitas/El Conquistador is a large resort located in Fajardo. Nice pools and a fun water park on resort grounds. However, it is not on a beach and their beach option is to take a ferry ride to their private island. We like it there, but as maytrix said, you won't get much of the local flavor.
If it is your first time spending time in Puerto Rico and you only are staying a few nights I would recommend something in the greater San Juan area. Chill at the hotel, explore Old San Juan and recuperate from your charter.
