Under really high pressures you might be able to make bubbles of xenon or radon (don't try that one at home!) sink in liquid lithium. It is opaque however, so you would have to study their behaviour with X-rays or ultrasound or the like. But I would expect them to sink and then coalesce like any rising bubbles.

A pressure of some thirty or forty atmospheres should do it for xenon.

Hm. at pressures not much higher than that you might manage it in water.

Might be fun...

I might be wrong here, but try this:

Go up in a hot air balloon; pour a glass of fizzy lemonade; cut the ropes attaching the basket to the balloon; as you cut the last rope, quickly invert the glass; witness the bubbles going downwards; enjoy the sight for a few seconds and then deploy parachute.

Don't try it in a built-up area because the falling basket might injure someone.

Alternatively, how about putting the lemonade in a bucket and whirling it around? The bubbles should go down but they might be hard to watch.

In a simplistic way this effect can be viewed at your local pub if you pour a Guiness. The bubbles from the foamhead appear to be going down to the bottom of the glass. They seem to be behaving as waves cascading down the sides of the glass.

*note: yes they may be rising in respect to their surrounding liquid but the overall effect is bubbles going down, which is quite fun to watch!