I spent a week in Newfoundland in 2003. Some tips:
(1) The TCH isn't all there is in NL. The most interesting detour is the Viking Trail, through Gros Morne National Park to various Viking-related historic sites at the northern tip of Newfoundland island. A short but pretty mandatory detour from St. John's is to Cape Spear, easternmost point in North America. Also a variety of colourfully-signed scenic loops from the TCH, one of which will take you to the town of Dildo (I bought a hat there). Another will take you to the jumping-off point to the French possession of St. Pierre et Miquelon (I couldn't do that due to lack of time).
(2) I got to NL from Nova Scotia via an overnight ferry from Sydney NS to Argentia NL, which takes you almost to St. John's and avoids a lot of backtracking over the TCH. It's possible to do the triangular route from Sydney to Argentia to Channel Port aux Basques (western tip of Newfoundland, with frequent ferry service to Nova Scotia) in reverse, but that might mean a long, boring trip from Argentia to NS in the daytime. Play around with the ferry schedules and see what works best for you. When I went, NS-Argentia ferry service ran only three days a week in the summer, so I suggest you make that reservation first and plan the rest of your trip around it.
(3) In Nova Scotia, the Cabot Trail is an extremely scenic loop off the TCH you should definitely work into your schedule if at all possible.
(4) If your car requires premium fuel, there will be some long stretches where only regular unleaded will be available (like through Gros Morne National Park), so bring some octane booster.
(5) Do not underestimate how long it takes to get around Newfoundland. The roads are only a little worse than other provinces, with plenty of 90km/h and 100km/h limits (nothing posted at 110km/h I can recall). But just the raw distances will chew up a lot of time.
For my trip photos from NL and elsewhere in the Maritimes, see
http://www.alaskaroads.com/photos-Newfoundland.htm