Do you have any money left to use? If so, how about doing a "thank you and farewell" end of year picnic. If the parent group could supply hotdogs, buns, and condiments, then have everyone bring a covered dish it could be a great way to bring everyone together one last time. You could then give a nice thank you speech to let everyone know how much their work over the years was appreciated.
We have a couple of local grocery stores and a bank that have big grills and will come cook for events-maybe you could find something like that in your community.
I'm looking for ways to thank everyone (parents, PTO members, teachers etc.) with little or no money since our school is closing at the end of this school year. I don't feel right letting all of these people go un-thanked for their years of support and dedication when the doors close in June.
