Every tourist gets told they MUST visit Maracas beach... but in terms of beaches, I think an extra 10 minutes to Las Cuevas is definitely worth it.
For hiking and/or nature trails, there's the Paria waterfalls (I'd advise you take along a guide or someone who's done the hike before), or for an easier time of it, the Asa Wright Nature Center.
For turtle watching, Grande Riviere and Matelot are excellent, but you'd need to check the time of year. Grande Riviete has a lovely little beachfront villa that you can rent rooms at, to stay a bit longer.
I think La Vega in Gran Couva and "The Bamboo Cathedral" in Chaguaramas are fantastic areas for exercising, relaxing or simply having a picnic. If you are a plant fanatic, La Vega is an excellent place to see and buy a wide variety of plants.
This might sound real plastic but the best places for me to visit in Trinidad are the malls and Movie Towne! Sure we have nice scenic place to see but you can only take in so much of those and then you want to do some shopping!
The best places in Trinidad are often the ones that are the least commercialized such as our forests and trails. Maracas used to be a nice place but its not as pleasant as it used to be because everyone wants to sell you something.