If you are staying in Northbrook, then I would recommend Francesco's Hole in the Wall or its sister restaurant, Next Door Bistro. It might be pushing your $20 budget though, as the entrees probably range $15-25. I believe that they open at 5, are closed on Mondays, and cash only.
Francesco's Hole in the Wall
254 Skokie Blvd.
Northbrook 60062-1612
847-272-0155
Next Door Bistro
250 Skokie Blvd.
Northbrook 60062-1612
847-272-1491