Leave the trunk. Cut it to the height of a chair and cut it level across. Put a board across it and use it as an outdoor chair. If not that it will make an excellent planter.

If the trunk is quite large and straight you can call a sawmill to come get it and it can be used for flooring, etc.

Cut it up with a chain saw or hire someone to cut it up in fire wood.

use it for firewood

We very unfortunately have to cut an oak tree down that is leaning precariously toward our house. We want to use the tree to its maximum and not waste it. The branches we'll use in Winter for firewood but what can we do with the trunk that will be worthwhile and not super expensive?