(1) Pictures or designs.
If your proposed project includes UI, then consider including a rough sketch of how you propose the interaction should work. What goes on the screen where, in concrete terms. Pictures don't be perfect and beautiful etc. But a good sketch that shows what you'd like the user to see (and what they can do) - that can be really helpful.
Similarly, if it's an API-only project, consider sketching out the API in text or some sort of architecture diagram. It's very helpful to show a rough design.
(2) Demos and screenshots.
If you've coded a pre-project or a demonstration for a project you're proposing, then make sure it can be run! Ideally, post the demo on a public URL so we can just use a hyperlink to run it online; otherwise, provide detailed build+run instructions for us, and possibly a screenshot. We'll also look at the code, but having a running demo - again, very very helpful.
(3) Dates and milestones.
It's very helpful to include a breakdown into subtasks and milestones with approximate dates. What do you think is the first thing to solve in the first few weeks? What's a reasonable goal for the half-way-point? Thinking through possible sequencing now will help prioritize, understand dependencies, and scope the project for the summer.
These tips are not Pencil-Code-specific, just things that I think help make a strong tech project pitch. We'll be aiming for projects that seem both awesome and achievable, so designs, demos, and milestones all help with that.
I'm really excited by the ideas here so far! I am looking forward to reading through your proposals.
David