I thought the drifting suggestion was pretty good. Actually, this is what dictionary.com said about it.
http://dictionary.reference.com/search?q=balloon
v. bal·looned, bal·loon·ing, bal·loons
v. intr.
1. To ascend or ride in a balloon.
2. To expand or swell out like a balloon. See Synonyms at bulge.
3. To increase or rise quickly.
v. tr.
To cause to expand by or as if by inflating.