what can I use this for??

I would say they will be ideal for seeds and cuttings.as you say like a mini greenhouse. just make sure you keep the seeds & cuttings watered.

Certainly 'potentially useful' and worth bringing home. You have already made the two obvious suggestions. How about burying one in the ground as a small water feature?

I could get two full seed trays under each one. could they cover tender or young plants over winter? into their permenant postion now?

I have 3 large thick tubes of the same material which i,m going to make a water feature with.

Can you drill large holes in them and paint with a coloured paint (like stained glass) they would make very unusual planters standing on the same coloured feet?

They could cover tender plants over the winter, but is there a lid you can prop open a bit for airflow on milder days?

some good ideas there..
theres no lid on them (they originaly stood on plinths for shop displays).
I saw those victorian bell jars but not sure what they used them for and thought these could be used to the same effect.

I think you would get your hands snapped off for victorian bell jars? They were used to protect plants. I have a smallish modern one I use to cover food looks great.