I have a few spare glass bottles and want to explore what decoration I can make with them, without just painting outside them.

Preferably safe, in case of an accidental breakage or leak. Preferably indefinite shelf-life.

Some ideas:

• Density layering experiments ('rainbow in a jar', maybe suspending an upside-down toy boat in a dense clear liquid with a blue watery liquid on-top)
• Phosphorescent liquids to glow in the dark (pref with no UV lamp needed)
• The oil+water+antacid+food coloring lava lamp, but the effect is probably too short-lasting to have running for a few hours