Arthur C. Clarke speculated about "oxidizer-fuel" jets flying in the methane atmosphere of Titan (Actually, Titan's atmosphere is mostly nitrogen with only 3% methane).

A more fun idea IMHO is a nuclear powered ramjet. They proposed nuclear powered ramjets back in the 1950s. A nuclear powered ramjet could theoretically fly for years without landing or refueling. The thing would be the ultimate weapon. It'd be programmed to fly low over a target city and raster scan it into destruction with its shockwave. The concept was a no starter because the ramjet's wake would be radioactive. However a nuclear powered ramjet would be cool thing to fly around on Jupiter or Venus.

Gary