If you are lucky enough to actually go in one of the balloons then I'd recommend the widest lens you have. Are you shooting FF or crop? the 17-40 is a minimum on FF, my favourite ballooning lens is the 15mm fisheye on FF. Here is an image I took in central Florida, it was with the 15mm and I de-fished it Lightrrom.If you are shooting from the ground I'd do the 24-105.
Oh nice! I've always wanted to take pictures up in a balloon! Only one lens?? well... Just my thoughts: UWA, telephoto and maybe a tilt shift? Depends on the scenery.. You could get some interesting shots of the landscape below.. Making them look miniature? I would take those kind of shots with the TS. Wide general purpose for maybe inside the balloon.. Occupants/self photos. Tele for other balloons and their occupants. And for things far below? If I was forced to take one lens, UWA. Have fun! Oh and wear a strap like a blackrapid. Lol