Seriously, my problem was that I once again figured signage would point the way. I've lived here four years; you'd think I would have learned to check the map before trusting in the existence of signage. (And checking aerial photography before trusting the map, but that's a rant for another day.)
Coming into Sabana Grande from Ponce on the Expressway (2), take the 121 exit. Do not foolishly believe that 121 will have to meet 120 at a defined intersection. Instead, follow 121 until you pass the stores in the middle of town and the street jogs a little to the right. Now you are on the actual street grid. Turn left on the street named "Rafael D. Milán" and go down two streets, then turn right again. Drive until you hit a T intersection, and if you are in the right place, you will see a sign (!) for Maricao, the town, to the left. Follow that, and where it leaves town, that's 120. Freaky!
Now. There is no trail map for Bosque Estatal de Maricao, but don't let that stop you. It's a nice park and an incredible view from the mountain. Drive up 120. At kilometer 14 there is a lookout tower, a fine stone edifice built by the CCC in mid-century. This is Montaña del Estado, so named because from that tower you can see both the south and north sides of the island, from Ponce to Cabo Rojo to Mayagüez to Guajataca. It is breathtaking.
Continuing on 120 to about kilometer 17, you will find the DRNA park facilities. The ranger is named Adrian, he's officially on duty from 7:30 to 2:30 on weekdays or until 3:00 on weekends, and even though there is no trail map, he will show you the trail heads. Tell him Michael from Ponce sent you. He'll get a kick out of that.
There's the ruins of a schoolhouse dating from before the park's opening which is nice to look at, and the trails themselves are relatively well-maintained old secondary roads. The view is great, and the flora typical mountain-jungle. Maricao has a number of indigenous bird species, but you'll have to get there early in the morning to find them.
The trails are actually relatively visible from aerial photography available on Google, so if I find the time (hahaha) I would actually like to compile a trail map. If I do, I'll post it. Another notion I have is to write to DRNA and actually ask for one. I don't know if that will work. We shall see.
At any rate, Maricao is a nice park to visit to kill a few hours hiking. We spent about two and a half hours walking downhill on one trail and back up, and hadn't reached the end, so unless you're a hiking maniac, you're not going to run out. Worth the trip.
The poison ivy to worry about in Maricao is Comocladia glabra, called carrasco or guao. It's a bush with the usual polylobate leaves, but the leaflets look a little like holly. If you don't leave the trails, you should be fine. Actually, C. glabra is common throughout the wetter parts of the Caribbean (i.e. the northern and mountain parts of Puerto Rico) so if you're going to be hiking here, it's a good idea to learn to recognize it.