I'm a complete nerd for a functional DIY project, so when I was walking around my local Michael's art store I came up with making a little chalkboard that I could use to write little messages on. It was a VERY simple project that anyone can make and can also be made as large as you'd like. My project below came out to $15 with everything included (I already had the glue gun, but you can pick one up for next to nothing at any local art store or W-Mart).
Step 1: Purchase a frame in any size
Step 2: Purchase a basic chalkboard that is about a 1/4" smaller than the frame (just make sure that the chalkboard is larger than the opening for the picture)
Step 3: Using a hot glue gun (for much larger/heavier projects use small screws for the four corners) dispense glue around the wood frame of the entire chalkboard and press down onto back of frame until dry (will take a few seconds)
Step 4: Attach either a frame hook to the back using the tiny nails the kit comes with or for smaller projects, use the tab off a can and hot glue it to the back of the frame (low enough so that it doesn't show when hung)
Step 5: Flip it around and check out your awesome 5 minute project!
Step 6: Hang on wall
Step 7: Write a note to your guests (this pic was obviously taken a while ago)!