Clear ice - boil it to get all the dissolved air out, put it in container, exclude air, freeze.
round ice - as above, but use two identical semi-round bowls (mixing bowls come to mind), freeze, unmold, put together with a bit of water in between, refreeze, voila!
Yes, boiling the water works. It really does give you crystal clear ice cubes without the white middles.
I once made INCREDIBLE center pieces for the tables at a dance:
- I boiled the water and then used them in the type of bowls you recommend: The upshot was that I had thick ice bowls with "holes" in the middle.
- When the water was kind of half-frozen, I filled the ice with holly leaves and berries picked from my yard (I should note it was a Christmas dance). If you're doing something similar, you could use colored marbles, sliced fruit, or whatever else fits your theme -- but let the ice freeze a bit so you'll end up with your items "suspended" in the ice -- you don't want them all on the top or the bottom of your centerpiece.
- I popped tea lights into the center of the glass bowls, and the effect was MAGIC.
The only problem: The centerpieces melted. I kept them in the freezer 'til the very last minute, and I set them on baby diapers (carefully trimmed to hide them under more holly leaves). Still, by the end of the dance, we had puddles.
Still, I highly recommend this as a very inexpensive, eye-catching centerpiece.
If you want to do it, one piece of advice:
Start early. I used Tupperware type bowls, but I only had enough bowls to freeze two at a time, so I needed a full week to complete the project.