Went to the Natl Book Festival this weekend. I highly recommend it if you are in the area. There are tens of authors - doing readings and signings, and lots of other stuff. The PBS kids tent had tons of coloring pages and other activities, and they had their own stage with Elmo and others. Also lots of characters (Clifford, Sid the Science Kid) around to meet your kids.
We love Mo Willems - author of Knuffle Bunny, Don't Let the Pigeon Drive the Bus, and the Gerald and Piggie series (among others). We stood in line 1 1/2 hours to get his autograph. That was a little nuts - and I wouldn't do the book signings again ( the lines were all insane) - but it was fun. The kids really enjoyed.
Another tip - do NOT buy your books there. The Borders tent to buy books had a 45 line to pay, and the books were not special - same as you would get at any bookstore.
Next year we will go and hear more readings and see more stuff - no autographs. Definately worth the trip though!