Which Plants??

That's a pretty big question, dude. Can you be more specific such as:

are there plants you have in mind already?
what are the specs on the tank you're looking to plant?

Java Ferns and all of its variations (broad leaf, narrow, needle, windelov) do not need to have their roots in substrate. Likewise for Anubias nana petites or other small anubias species. They can be tied to wood/stone. Wisterias and water sprites can be planted in the substrate or left floating. Some stem plants, if left floating, will start to grow more stems out of the nodes. The same ones, if you anchor the stem horizontally with a pebble, will start to grow new stems out of the nodes.

Like jbradt said, if you give us a couple names of plants you're thinking about using, then we can give you a better answer.