Diners, Drive-Ins, and Dives. The best way to search is through THIS site. Just search for the city closest to your destination and you'll find tons of results. This is how divine intervention brought me to one of the best sandwiches I've ever had at MELT: Bar & Grilled. MELT serves over 30 kinds of gourmet grilled cheese sandwiches. Cue salivary glands.
The environment is fun and upbeat but not too loud that you can't hold a conversation. It is decorated floor to sealing with kitschy knick-knacks, signs, and graffiti. Instead of waiting 30 minutes for a table, we sat at the bar where food is also served. There is tons of space at the bar and we had our food in under 30 minutes.
I ordered "The Firecracker" grilled cheese which consisted of a blackened Cajun chicken breast topped with avocado and grilled pineapple between melted provolone and toast. It was so good that I texted "OMG" to myself. The french fries were among the best I've ever had as well. The plate also included a sweet cole slaw that was dry (not mayo based) and delicious. See photos below for more (click to enlarge).