So...it can't be in the tank....and it can't have anything coming out of the tank/going into the tank? Sounds like a very tall (read: impossible) order my friend.
A canister filter's still going to have a pipe going into the tank. In the case of a canister filter, it's going to have two pipes. Two!!!!
What is the goal of your filter going to be: mechanical filtration? chemical filtration? water movement? Those will all affect your end goal. A canister filter will affect all three, since it will mechanically filter out detritus, and you can add chemical filtration, and position the return pipe so that it makes water movement.
A sponge filter can provide mechanical filtration and water movement, and if done correctly, can be successfully concealed by dense plant growth. Adding in a powerhead instead of air will help with more directed water movement.
A box filter can function the same as a sponge filter, and you can also have a compartment for chemical filtration as well.
So many fish/plants/inverts to keep, not enough aquaria.