Hi,
besides the places Steve P. mentioned, the shop of Sam exists which can be found in the town center. It is the small hut just before the Restaurant Great Lime Inn.
Sam charges ~60TT a day and it might get cheaper when renting the bike for a longer period. Against an extra fee he delivers the bicycle to some other villages he visits during the week anyway.
Another option is Tobago Slow Leak Mountain Bike Tours in Grafton.
My description to find them:
http://www.alewand.de/fotos/tobago/mountainbike.htm
They offer also tours and have the better bikes. These are some nice guys who chose the tour according to your wishes and capabilities. For them the nature experience is the most important.
Some cycling routes around Crown Point:
Flat: 1. Everything directly in Crown Point. Just explore the area.
2. From Crown Point To Plymouth: Take the Milford Road to Scarborough (the only part which is not so nice). Turn left into the Golden Grove Road. Buccoo Road. You pass the golf course. Grafton (visit the bird feeding place, steep road). Black Rock. Plymouth (Mystery tomb). The same way back. Additionally, you can visit Buccoo.
By the way: Here is my version of the Mystery Tomb:
http://www.alewand.de/fotos/tobago/fo88.htm
3. From CP to Scarborough. Next to the main road is enough space to use the bike without being in danger, but it is not a nice cycling tour.
Steep: Turn right after the Mt. Irvine golf course to Luise Kimme's house. Continue to Bethel. Just before reaching Patience Hill, go down to Black Rock.
Steeper: When exiting the Golden Grove Road, turn right onto the Buccoo Road. After a while turn left and continue until you reach the Auchenskeoch Buccoo Bay Road (I like this name). There is a road between the road to Bethel and the Dunvegan Road. You will encounter this very steep part:
http://www.alewand.de/fotos/tobago/fo12.htm
Continue and choose to roll down to Black Rock, Signall Hill Rd. or Orange Hill Road. Back to Crown Point on the main roads.
If you choose the Tobago Slow Leak Mountain Bike option, you might visit the Highland Waterfall next to Les Coteaux. Look at their web page
http://www.tobagomountainbike.com/trails.htm
and choose a tour from "Beginner" to "Extreme". And yes, they are not expensive.
PS: You will have a puncture for sure. Do not forget the fiying kit
Greetings,
Achim