I was recently doing some "out of track" glitching on version 1.00 of GT5 and it's amazing what you can see. I'm sure some of you will know most of what I'm going to put here, but I hope I tell you about something new.
Circuit de la Sarthe
There isn't much tht is modeled away from the track, but you can see some houses and the rotaries off of the Mulsanne straight. Bugatti Circuit is modeled completely but not solid so you can't drive on it. I was able to make it look like I was driving on the first part before it dips down. It does become solid near pit road and if you get too close you will auto drive into the pits by taking a little off shoot exit. There is also a parking lot near the start/finish line that has full physics for sand, grass, tarmac and curbing. I was really surprised by that.
Daytona
On the oval version the inside road course is solid without the barriers. This means you can do laps around the infield short course. You can't get out of there once you enter though. The outside portion of the infield is well detailed and you can get back to the oval from there. I was hoping to drive on the go-kart sprint track that is in between turns 3 and 4, but it blacks out most of it when you're there. I went to the motorcycle course part of the track and did a bunny hop when I exited back to the oval. Behind pit road is well detailed and I even went and parked in victory lane! This is an image from Google Maps of the go-kart sprint track. The tarmac for the final turn is not there in GT5.
Fuji
PD modeled details for miles away from this track. You can see roads and power lines for a long way. You can also see the training course just outside of the track as well as the famous banking. I wish I had some pictures for this one as there is a lot to see.
Indianapolis
On the oval version the road course's details aren't smoothed out yet and it isn't solid. There are some roads that lead around the speedway. (just textures) I liked seeing all the little food places and the gift shop. It looks like there is also a golf course there.
Monza
PD created a little more than half of the abandoned oval. They did not model the backstretch nor the parts of the turns leading to it and from it. Actually, they might have modeled part of turn 2 but they didn't model turn 3. It is not solid. I have also driven on the road that is a part of the short version of Monza. It doesn't seem to be solid, but is at surface level.
Special Stage Route 7
The cloverleaf's on and off-ramps near turn 1 are solid, but you can drive through their walls. Route 7 continues straight for a little bit after that cloverleaf. It is flat, but not solid, as you can drive through the center guard rails. The overpass for the cloverleaf might be solid. You lose road textures after driving down the bridge for a few feet, and there are some light poles that go beyond the bridge. There are some pairs of light poles that seem like they would outline roads around the cloverleaf area. Halfway down the track are two exit ramps that are solid. After the tunnel are two more exits that are not solid. They run above and parallel to the track. There is a road that crosses over the tunnel exit that connects them. The toll booth area on the other end of the track is solid, but the toll booths are not. It's still funny to sit there and "pay the toll" before getting on the track.
Suzuka East
If you get out of Suzuka East you can turn laps around Suzuka West. Suzuka West's walls are not solid though. This also means you can jump off the bridge. There is a parking lot just behind Suzuka West's pit road that you can drive down into. There are also the roads that lead to the Suzuka complex. When I followed those roads I was able to find
this sprint track. It was very poorly detailed however, and I could barely make it out.
Top Gear Test Track
The runways go for quite some distance and you can see some buildings. The main runway is solid, but the run down one that crosses it is not. That doesn't matter though as you'll be at ground level there anyway. There is a building and, either a trailer or a dumpster on it though. I There are some perimeter roads that aren't solid but the majority of them are at the surface level of the track. They were pretty fun to drive on. The main road that with the dashed yellow line down the middle is not solid except very close to the track. You can also see some parking lots and the building on the main road.
As you can see, if PD wants to add them in the game, they should remodel them 
