Since I'm taking the same trip later this summer, I did a little Chowhound reseach for us. The best I can find in or around Lafayette seems to me to be:
1) Riverside Inn - Cajun
2) Old Tyme Grocery - PO Boys - Particularly Shrip
I guess it depends on your time and travel crew.
Also, you go through Boudin country, so check out this site for more than you ever wanted to know on Bayou Boudin.
Prejean's near Lafayette, Louisiana.(pronounced pray-johns) Very good cajun cooking. A large place , many evenings with live music. But, cooking is at a high level. Still remember the crawfish bisque I had there....I tried many times to duplicate it and never came close. It is located on I49, about 2 or 3 miles north of the intersection of I10 and I49.