Suggestion on choice of lens for hot air balloon?

You are probably not going to be higher than 500 feet, so you could pace that out on the ground and try different lenses for angle of view. If you want scenic coverage, I agree with the 28-85. If you are only taking ONE lens, that would be the one to take. If you want to zoom in on things from that elevated point of view, the 70-210 would be nice, but that's not my first choice for the only lens to take.

Keep it as light as possible :-)

This is not from experience but from my expectation:

Definitely take an ultra wide angle to get in fellow passengers with the context of the balloon and the view all in focus. Asking fellow passengers to step back a bit, bit more may not go down (!) too well.

That will also be good for taking shots as they prepare the balloon for take-off starting from the shapeless bag on the ground to the final thing.

The 28-85 would be a good general purpose workhorse.

What time of day? Good light for the whole flight? A fast prime 24 or 50 would be useful.

Finally have everything packed back in a well padded bag for the landing - can be a bit bumpy!! Last thing you want is an SLR swinging around your neck!

When the temperatures of both inside and outside are same, then we cannot say that the air inside the balloon is hot. The hot air balloon works because of the temperature difference. You have to think of other ways only. The helium balloon goes up, even though, the temperature inside and out side is same. In hot air balloon air is made to have less density by heating. In helium balloon helium by its very nature is less dense than air.