San Juan is a good area to stay as you are in close reach of Port of Spain and other main areas. Maracas Beach is about a 45 minute drive. There is not much to do if you were to walk around San Juan and it is not recommended to be walking out past 6/7- the majority of places have closed by this time too. However it is a great base.
I really do not like towns. The Caribbean Villa is a home away from home. About San Juan- you can find a Western Union in the Centre next to TruValu supermarket in the Croissee, phone stores roti shops. You ought to taste a Trini Roti. I wrote a jingle on that for promotion. I do not recommend fast food, so there is a shop for creole food, a more nutritious way to go. There are plenty stores for shopping if that is what you like, but keep a price watch. In some business places like B Mobile, the service is very poor. They are not customer friendly. I intended to purchase a phone. When I looked around for the Indian girl to serve me after she had served an Indian customer, she had vanished like smoke or steam into the atmosphere. I never went back there. I went to another shop to get what I needed from Digicel. Transportation is easy in the daytime as there are many maxis or mini-buses and route taxis and the odd private car of which you should beware. Some taxi drivers are extortioners or crooks, like one whom a local stopped to take me to Caribbean Villa, only about 2000 metres away, but my phone's battery had run out of power so I could not use the Google maps-recommended, especially for drivers who do not know their own town or may be trying to trick you they had trouble finding the place so they overcharge, even while driving past the destination. That guy made me walk back to the guest house with my bags and still charged me $6.00 for the trip, which is only worth $3.00. he is a crook, so keep your eyes and mind focused.