The El San Juan also has a casino if you're into games of chance. So do many of the other hotels in the Isla Verde and Condado Beach areas, as well as the Wyndham Old San Juan.
If I had such short a time in San Juan, I'd stick to the shops, restaurants and historical buildings in Old San Juan and skip the beach areas. The ships dock right in Old San Juan and if you get tired of walking, there's a free trolley that circuits the area. I'm not sure how late it runs, however.
You might enjoy walking up to the beautiful fort, El Morro. The views won't be as spectacular at night, but it's a pretty place.