It took me a while and some aggravation, but I've found all twenty.
This is definitely spoiler stuff, but I've got the list below:
Note: all balloons are off the beaten path and definitely not on paved roads or city street stones. They are always on lawns, sand or dirt.
Also note: you're supposed to bust them by driving near or over them.
Balloons:
On the beach there are four: One near the farthest island on the ocean side, one on top of the shorter island (use a blue ramp to get up there), one at the camel rock and one way back past the resort near the tunnel - not up by the tunnel, but on the beach near the cove where the tunnel runs above.
On the hillside above the tunnel.
If you cross the path that leads to the ridge and bike along the narrow ledge there, you can find a balloon at the very end of it.
There's a balloon in the sand (or is that salt) of the dry lake bed in the same spot as one of the flags from earlier levels. In case you never found that flag: to get to that lake bed you have to go in the cave in the field above the path just as it reaches the ridge.
There's another balloon in the field of the widest part of the ridge near the entrance to the cavern that leads down to the lower field using a metal spiraling ramp.
In the field where the lower entrance to that same cavern sits at the top of a dirt ramp, there is a ledge that begins to the left (if you're approaching the mouth of the cave from below) ride along that narrow ledge to the end and there will be another balloon just before where the path below goes through the hole in the rock.
There is a balloon at the lighthouse. It's past the building, right out on the point.
Travelling along the paved road, past another hole in the rock to the right is a narrow field and a balloon up at the end of it.
Across the archway (still on the paved road) is another field on the other side of the inlet. There is another balloon there.
There is a balloon behind the temple in the ruins.
There are three balloons in town: one behind a large building near the water on the lighthouse side of town, one in a grassy alley in a building near the bridge side of town (on the water side of the wide main street) and one in the park above the fountain.
On the beach/resort side of the suspension bridge, near its footing, there is another balloon
There's one by Duckling lake high up the hill. To get to this point you have to go under the waterfalls at the lowest level and pedal your wii little feet off. As you finally crest the hill by the lake, turn around and find a field. At its far end is another balloon.
Behind the castle (between the castle and its wall) is a balloon.
At the stonehengey monument on the plateau above the castle (you can go either way around the ring) is the last one.
That should be twenty. Finding them took several sessions. I have not yet found and burst them all in one session. I'm hoping if I do I will get some kind of reward like maybe one of those doors in the ruins, castle, resort, etc. opening.