Out on the tiles...with speaker spikes.

Coins are good and almost invisible, use Canadian coins, very well made but way cheaper, each one will actually cost you 0.157349 USD at today's exchange rate. Hockey pucks are good but highly visible. Try it without any spikes, you may not hear a difference and may start a reversal of trends in the audiophile community. From the Canadian mid-fi trenches. Good day.

I have a beautiful black tile floor. I use Walker 1/2 inch discs to protect it. A bit more expensive than the other suggestions, but definitely worth it.

If your room is "on the slab" (concrete) you probably don't need spikes. From what I've read, spikes are best used on floaty, loose (i.e. wooden) floors or to provide acoustic isolation for us apartment "dwellers".

Linn used to make little feet for use underneath spikes, but I can't for the life of me remember what they are called. One of my friends uses them and they work very well. They seem to keep the speakers very stable. I would think you could pick up a pair for about 50 bucks.

If you've got the patience and access to the right equipment (I do!), you can cut aluminum round stock to the thickness you require, then countersink (a standard drill bit will work just as well) to the depth you want. The aluminum is soft, and easy to work with. Probably less abusive on your tile than a coin would be also. Then go to Home Depot or Lowe's, and purchase some felt dots to stick underneath. Although I much prefer the acoustics of a dead floor-- Carpet, with a heavy pad beneath.