That's pretty great (hauling home a dead tree trunk), its the sort of thing I would do. Do you know what kind of tree it was? It looks like maybe a cottonwood.
Hollow tree trunks are great planting holes. The problem is that they rot out quickly when you fill them with dirt. Can you find a pot that will fit in the hole? Or line it with plastic (remember the drain hole). That will make the tree trunk last longer.
What to plant? It depends upon how big the pot will be but a summer annual like Million Bells would be pretty. Or one of the decorative sweet potatoes. If you decide to just plant in the hole, you would have more root space but your "pot" will quickly rot away.
If you want to plant a perennial, I would suggest a "stonecrop" succulent or an ivy. Both stay green all winter.
CDsSister said:1. Are you planning to grow it indoors?
2. What kind of light will it get? Indoors or outdoors.
If you plant it indoors then you will need to find an indoor plant.
If you plant it out doors then that opens the possibilities pretty wide open -- but the light source (shade, full sun, part sun, etc)