I have a topiary tree that I decorate differently for each season just by changing out the top with a new Styrofoam ball (and fiddle with the bottom a little too) but for Halloween I used a hair band with a witch hat attached to it and added seasonal flowers
for Christmas I unwrapped the bottom and added poinsettias to a new Styrofoam ball.
Now changing out the season will be easy as switching out the top of my little pot!