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
100D, EOS M, EF-M 18-55mm f3.5-5.6, EF-M 22mm f2, EF-S 18-55mm IS STM f3.5-5.6, EF 28mm f1.8 USM, EF-S 10-22mm f3.5-4.5 USM, EF-S 60mm f2.8 macro USM, EF 40mm f2.8 STM, EF 85mm f.18 USM, EF-S 55-250mm f4-5.6 IS, 430 EX II, kata 3n-1 22, triggers, lighting, stands, remotes, etc...