Jellyfish

That is exactly what I thought back then, so I took a stick instead.

I would not touch these. Can be poisonous!

Thanks for your suggestion Viv!

If you open the Wiki link Kya in the I.D suggestion and then scroll down you will find a photo on the right hand side of the page with a link to Scyphozoa. Looks very similar to your spotting but it is ONLY a suggestion : )

Probably. I tried to turn it upside down, but in vain.

could be a jelly fish with the tentacles enclosed inside the cap.

I assume, I found this on the beach not far from my resort hotel, I took a piece of wood and stroke on it and the stick bounced back like it was made of plastic.

Jellyfish?