This is the best idea!! I want one for our home! Grab one of these Large Mark Your Travel Prints for Only $3.75! (Reg. $18). Use colored pencils for a soft look. Erasable Soft Highlighters (love the erasable part) are another great option.
Use tiny stickers and place over each state. Use a fine point artist pen and mark it with the date the state was visited. Cute simple doodles are a fun option too!