Think about the types of play that you enjoyed as a child. What made these places memorable? Recent research has suggested that natural play spaces are more memorable, with the opportunity to manipulate and shape the play environment. For more on this, see Nature-based Landscapes for Play.
Browse through the Inspirational Projects, and the Play Elements sections to get a sense of what is possible. Start making lists and doing sketches, and designate a team member to create rough sketches or gather images that are pertinent to your project and can be used to communicate your ideas to a designer later on. You can either create a physical space, such as an “inspiration board” or a file folder, or choose to store this inspiration in a digital format such as Dropbox or Google Drive.
The Boston Schoolyard Initiative is a project that is similar in scope, and based in Boston. They have developed an excellent Outdoor Classroom Design Guide, which we highly recommend reading. In addition, their Schoolyard Design Guide may also provide some helpful hints at this stage.