[Why] I did this one a few weeks ago right at the beginning of summer for our graduation weekend. I hadn’t really used HDRI lighting yet and wanted something that looked legit, so I tried this out and got a pretty cool result.
[How] For the balloons I used a sphere and put a taper deformer in it. Then added a few other shapes for the nubs at the bottom. I put them in a cloner and attached a random effector to it wouldn’t be so grid like and uniform. Since cloners suck up so much render power, I converted the cloner’s state to an object and it saved a lot of processing power. Since the balloons were part of a cloner and not an emitter, I was able to give them a rigid body tag. They don’t use it, but dynamics are active. Finally, I changed the MoDynamics gravity of the whole project to -5 and that gave it the float. Very simple, but pretty nice. Here is a still of the video loop I created for Graduation at MSM.