I think you would be pleased with the Sugar Rush Café. Along with some delicious breakfast and lunch offerings, they serve up a pretty serious tuna melt. The chicken & spinach melt is very good as well.
Try a cafe latte or mocha. They use Inteligentsia coffee and brew a decent cup.
Sugar Rush Café
900 Valley Blvd
Walnut, CA 91789
The area is somewhat industrio-desolate; here’s a map link:
If you are willing to drive a bit further east, Corky's in nearby Rancho Cucamonga. http://www.corkyskitchenandbakery.com... does a fine tuna melt.
And if you go, get a slice of the German Chocolate Cream Pie, or a whole one to go. It's well worth the additional calories.