Other than giant airships: how about just novelty/party balloons that never out-gas & sink? Re-positionable ceiling decorations? Indoor microblimps? If such th

Other than giant airships: how about just novelty/party balloons that never out-gas & sink? Re-positionable ceiling decorations? Indoor microblimps? If such things escape into the atmosphere, you've got a floating-garbage problem... as described in Stephenson's Snow Crash.