I hear this all of the time from people new to PBL. They really think that there will be a dearth of ideas and they will struggle when it comes to planning a PBL unit. In fact, when you have standards that you are trying to plan from there will be ideas.
Standards are the key. Until you have taught at least one school year with standards based PBL then I do not recommend creating a project followed by adding standards to it. And, even then, the majority of your PBL units should be standards based.
There are several reasons I could list for this but the main two reasons are, first, that by starting with the standards teachers guarantee that they have thought about the curriculum and, second, this is where the ideas are going to come from.
Teachers have been creating problems in mathematics, labs in science, writing themes in English, and the same type of things in every other course since we have had teachers. The idea here is that teachers are good about taking standards and creating tasks for students to do in the classroom. With PBL, teachers take that up a notch and they add an inquiry based atmosphere to the classroom. That's really all it is to this stuff.
For example, I had to teach parallel lines with a transversal in geometry class. I just happened to see an article about the rice business in South Texas and the canals that are used. Suddenly I envisioned parallel canals with crossing canals. So to take that to the next step I had the students design barges to carry rice through the canal system. They got to be creative and design barges and these barges had to be able to turn a certain number of degrees so they could navigate from any canal to any other canal.
Another example: my co-teacher and I had seen where a teacher had created pieces of art with her algebra classes using basic algebraic functions. We had to teach the idea of functions and suddenly we had a project where students had to take parent functions and create artwork.
In each of these examples there was a need to teach certain standards and the project idea came from that requirement. Here is a post I wrote a year ago that talks about getting started. It is one of a four part series on jumping into PBL for the new year. Feel free to read that series if you are thinking about getting started in this process. You can do it and, with the standards as your guide, you will have project ideas to work from.