Carrie
The town of Corinth isn't anything special, but the ancient site is great. There are two parts
a) the roman ruins which are quite extensive and well worth a visit.
b) acrocorinth, the old, higher fortress. This is huge and, if you climb high enough within it, you can see both ends of the canal.
There's a nice photo and some information here
http://www.panoramio.com/photo/8687938
The roman site is around 30 minutes outside the main town by local bus. Acrocorinth is a good 30 minutes walk up from the roman site. We got a taxi from the roman site to acrocorinth and then walked back down.
There's a taverna by the gate of acrocorinth, but I doubt it will be open in March. There's also a taverna near the roman site but the food wasn't great and was overpriced. Better to go back into town or take a picnic. We spent a longish morning doing both sites, and still felt we could have spent more time on acrocorinth (which we had entirely to ourselves on a sunny December morning.)
BTW, Nafplio is a great choice. Beautiful little town, amazing venetian fortress, and good restaurants.
For accommodation there it might be worth checking with Acronafplia, who have various buildings scattered around. They would generally be pricier than you are looking for, but might do you a deal out of season.
http://www.pensionacronafplia.gr/hospitalityen.html
Have a great trip.
Alan