What PP said + Take some pushbikes!
You can get round the course while the roads are closed. If you drive or ride a mo’bike to a watching spot, you’re pretty much there for the day.
On a mountain bike/crosser/tourer you can make your way around the course, pick a few vantage points and nip around on and off road. south and West of the course are specially good, and there’s an old railway line that follows round not far from the course. Start at quarter bridge, watch the leaders come, though ride up to Union Mills (watch from the inside of the track by the bridge as they come over the bridge and lay rubber blasting up the hill.)
Ride up to Kirkmichael between races, watch from there, up to Ballaugh Bridge, then stop for pints at the Sulby Inn… You can time your stops to watch the leaders come through. You can find a friendly looking Marshall and ask nicely to nip across the road between races, bash through some footpaths in the woods on the inside of the course and pop out at at Greeba bridge… Pack a bag of beer and tape a radio to the handle bars and off you go!