I agree with the previous reply in that you don't need to spend a full day in Porto Venere. A few hours and lunch should be adequate. On the return trip, stop in Riomaggiore and visit the other C.T. towns by train. If you want to see Corniglia you'll either have to walk up the long stairway from the station or pay a few Euro and take the small Shuttle.
I'd suggest timing your day so that you end up in Monterosso at supper time, as there are lots of good restaurants there. One favourite with many here is Cantina di Miky, which is close to the train station. Don't confuse that with the more upscale Miky restaurant. As far as "food questions", see what looks good on the menu of whatever restaurants you visit. I always enjoy something with Pesto, as that's the area where it was invented. If you like seafood, you'll have lots of choices.