Honestly, thinking of an idea is one of the hardest parts. Our troop has always done a lot of building stuff for projects (decks, fences, sheds, barns, stairs, pavilions, signs, gardens) so I knew I wanted to do something that was outdoors and construction.
I hate seeing stupid projects like painting a sign or painting parking spaces. You should be proud of your project and realize that its essentially the last thing you'll ever do in scouts. Make it worth it, because you only choose one project and for the rest of your life, you'll be telling people what difference your project made.
If you need a project, your best bet though is to talk to a local church or park and ask what kind of projects they need done. Or talk to other scouts who are working on their project, and ask for their project coordinator's email. You just have to be proactive about it.
The park I worked with set me up with all the materials I needed for free. They delivered the railroad ties to the work site, cut them to length with an industrial saw beforehand, provided a massive drill for the rebar, cut the rebar, a generator, gas for the generator, and every single hand tool known to man.
Only thing I had to do was buy the 25ft of rebar for the project and drop it off to them beforehand. I think the total cost to me for the project was like less than $35