Could you actually GROW small plants in them? Like in a terrainium? Ask at your local florist for suitable bulbs that will in season to add colour. You have to use special compost as there are no drainage holes, but your florist can help you, or local garden centre.
Add loads of green foliage to, for contrasting texture, but a large bulb, flowering from the centre would be different.
Have a lovely day.

Oh, if you are having those huge bubble vases, why not fish??? and coloured gravel???