A Christmas tree or wreath immediately comes to mind.
You may get some other ideas in this site.
http://www.delamar.org/gtshaped.htm
help writing a meaningful shape poem
2 answers
Here are a number of sites on concrete poems with suggestions
http://www.gardendigest.com/poetry/concr1.htm
http://www.gardendigest.com/poetry/concr1.htm