I put tropical cannas in my bog every year. They are relatively cheap, easy to plant, and grow to an incredible size in just one season. Plus - pretty flowers! I could pull them before they freeze and save them for the next year, but - lazy gardener. However, they are equally easy to remove at the end of the season which is also important - you don't want things that get so huge that you have to have an all day battle to get them out. I had an epic day with a clump of irises that I turned my back on for just one season - had I not gotten them out, I would have had an iris bog by the next year. And even though I think they're gone, they still pop up in a few spots every year!
So for me, it's always the three easys - easy to buy, easy to plant, easy to get out when the time comes!