This is a lovely program and yes, Comic Sans fits 100% perfectly :) haters gonna hate!

I imagine Comic Sans fits, but you may want to include a fallback to sans-serif for non-Windows users. I'm getting the default serif font, which is not a great match.

If you want a "casual" font then I'd recommend something that looks more like your logo or just looks more like marker/drawing board text in general. Fontsquirrel should serve your needs well.

Also, I'd recommend you to redo the logo digitally so you can overlay it on the background seamlessly - the mismatched grids look pretty ugly right now.