I keep my quarantine tank planted and running with a few small fish at all times. As tends to happen, my "quarantine" tank became a permanent tank it seems. Oh well, one can never have too many really.
If you don't want to keep a tank running, sponge filters are great choice. Keep the sponges in your cannister filter (if you have one) when you don't need them. When you get new fish, take the sponges and use them until the quarantine period is over. Boil the sponges and pop them back into the cannister until you need them again.