Be sure to turn left.

400 _is_ high speed, isn't it?  If it were me I would use 100 for everything
and use a tripod if needed.  Balloons don't move fast and it's not like you
run around a balloon floating in the sky looking for the best vantage point.

1) Polarizer - works best at ~90 degrees to the sun
2) UV blocker, jury is out on what works best.  B+W UV 415 is
  the best specified.  A 'haze' filter will work better than a 'UV
  protector' a la Ritz -- they are different.
3) Warming filter can help with pictures not taken at sun-up/down
  (actually a nap works best and saves film...) and can take some of
  the blue out of high-altitude shadows, 81A is common
4) A didi enhancing filter sometimes, somewhere, though I have
  found neither the time nor where.

Don't get your hopes up.  A polarizer will produce the most
effect but only at certain angles, luckily those are also
photogenic angles.

Find a good bar?

The same filters work in cloudy weather -- though the
polarizer will only work to remove some specular reflection.

Cloudy weather is pretty blue and so a warming filter does
help.  Clouds pass a lot more UV than visible so ditto a UV blocker.

Polariser.

Use a more saturated film, such as Kodak Elite Color UC 400.  

It will give punchy colours even on grey days.