As we know, gas giants do not have a solid surface, but on Earth we can float a balloon in air. Are there any plans to send balloon-like probes to gas giants so that they can float in its atmosphere and explore the gas giant longer?
I've seen it in viewgraphs, but not in a serious proposal. I suspect that there will be more traditional probes (entry vehicles on parachutes) to Saturn and one of Neptune or Uranus, since we haven't done those yet, before a balloon would be attempted.
While not a bad idea, there are several problems with implementing this in reality. For starters, scale: balloons don't tend to move very fast, and Jupiter is huge. Just from the top of the atmosphere to the first layer of clouds is nearly 1000 kilometers (10x Earth's atmospheric depth). As the balloon would dive deeper, other factors become a problem. Although Jupiter's atmosphere is not dense, there is a lot of it, meaning that temperature and pressure would quickly become a major concern for the balloon in question. The balloon might be able to take readings from the upper atmosphere, but would likely have significant trouble going deeper than the Galileo probe already did (about 1000km). Providing the balloon somehow survived this, it would eventually reach an environment that a balloon is completely unsuited for: liquid. The concept in the question that there is no solid land in a gas giant is not necessarily true. Deep within the planet's atmosphere, pressure builds and builds until the surrounding gas became liquid. There is no surface, but the pressure increases until it is defined as a liquid and not a gas. Theoretically, at lower depths, the liquid would eventually be under so much pressure as to become a solid, again without a surface. So while a balloon might not be an awful idea for exploring the very top of a gas giant's atmosphere, a more traditional, heat and pressure shielded probe (such as the submarine in Ben Bova's Jupiter and Leviathans of Jupiter) would likely be more effective at exploring anything below the top thousand kilometers of the atmosphere.
There is VICE (Venus In Situ Explorer) which is proposed mission to be launched around 2022 (if it gets approved of course). I think the two major approaches here are to use a balloon or airplane.
I seem to remember reading something about a similar mission being discussed to Saturn or Titan, but I can't seem to find any references.