Here are my thoughts, both places are good for half day/one day trip. However, I would not go to el Yunque at nighttime. (http://elyunque.com/trailmap.htm) The road is very narrow and dangerous in the dark. There are many sights to stop and see at el yunque, along with trails to walk. I gave you a link with sights on it, stop at Las Cabezas observation area, must see La Coca falls, Yokahu observation Tower, Juan Diego Falls, take the trail to la Mina falls, bring comfortable shoes and your swinsuit, as you can jump in the water (there is not clothing change area). The trail towards Mt. Britton is about a mile long, keep that in mind, as this might be time consuming if it has rained before. You can stop by the visitor center for some rest. On your way to Guavate, (http://www.discoveringpuertorico.com...of-the-lechon/) remember that it is only open on the weekends, usually after 11:00am Saturday and Sunday. There are many lechoneras along the way, do not stop on the first ones, just follow the road until you reach an area that has several of them next to each other, should be about 15 minutes or so, depending on traffic. I usually go to Lechonera El Monte, or the one across the street from it. Try tostones, pastel, amarillos, mofongo, mavi (a drink made from sugar cane and bark), and of course lechon with rice and beans, if you want beer go for the local one Medalla. Keep in mind the road is rather narrow and curves a lot. This is an excellent place for lechon asado. There is one in old San Juan, but i do not recommend it.

If you like beer, my favorite bar is Taberna Lupulo in Old San Juan, by calle San Sebastian, (http://beeradvocate.com/beer/profile/23956). A good restaurant to eat in Old San Juan is Vaca Brava, but you have to go in Early, as you can easily wait 45-70 minutes for a table. Eating in condado area can be expensive, I suggest any of the local panaderias, for a sandwich, try a mallorca.

Anyway, I could go on and on, but i gave you some ideas to try. Enjoy my beautiful island, as much as I enjoy SFO when I visit there, love Sedona, R&G Lounge in Chinatown, and Fishermans Grotto for cioppino and the sourdough bread from Boudin. I'm hungry now.