You've already painted over half your bedroom to turn it into a dry-erase board for your spur-of-the-moment ideas. If you'd like to doodle some with chalk instead of ink (because you miss all those gross powder on the finger things you got so used to in elementary school), the Rust-Oleum Chalkboard Paint should do the trick.
Designed for indoor use, you simply coat it over any surface to turn a patch of wall into a functional chalkboard. Once dried, you can use it just like the blackboards in your primary school back in the day, writing nasty stuff about your roommates and doing your best Bart Simpson impression.
The Rust-Oleum Chalkboard Paint can be used with most common surfaces, including wood, metal, plastic, paperboard, glass, concrete and hardboard. When applied, it creates a scratch-resistant matte finish with superior hardness that ensures it won't just peel off after a couple rounds with your rowdy kids. Similar to real chalkboards, you can remove marks on it with a chalk eraser, but you'll need some soap and water to fully tidy up. And just in case you're into hitting little plastic balls with a paddle, they also recommend it for resurfacing ping pong tables.
Amazon has the Rust-Oleum Chalkboard Paint available, priced at around $12 for each 30-ounce can.