I envy you. I am patiently counting down the hours to my visit to Tobago as a treat to myself before I start law school.
Totally visit Castara. It's so beautiful and the community extremely friendly.
As for nightlife, mostly liming at bars or Sunday School from 8PM in Buccoo, The Shade in Bon Accord or Bar Code in Scarborough. But overall, Tobago is best enjoyed by the seaside or at a great seafood restaurant. The seafood is amazing. I highly recommend Seahorse Inn. Have fun!