I recommend Mark's East Side - we recently enjoyed some great German dishes there, and there's the traditional American and fish specials too. Big portions, the quality of the meat was excellent, and the Provimi veal liver was especially noteworthy. Pretty extensive beer selection too. The owner was really cordial and took care to see that everything was good at our table.
Mark's East Side Dining and Coctails
1405 E Wisconsin Ave, Appleton, WI 54911
If you're near Brillion in the morning, Rudy's Place on Route 10 is a vey good choice for breakfast.
Hope you have a good trip.