Silk are nice, but in my opinion, live are just freaking awesome :). You can get hornwort really cheaply at any home depot in the gardening section, and if you want, you can let it flot or make a plant weight from a plastic cup to have it look like a rooted plant. The only problem with it is that if it doesn't like you or your water, the needles decide to fall off and make a huge mess in your water.
A less messy plant could be an anubias. Don't bury it's roots, just tie it to a rock or something.
Marimo moss balls are interesting, they sink and float.