You might find this site a starting point ...

http://www.cityvox.co.uk/visit_calais/sightseeing

but there isn't really anything of great historical interest in Calais that you can see on foot ... most Brits are there for the shopping and the beer and for the fact that you are 'abroad'. I would suggest you make sure that you find a hotel that is close to the town centre, and not in the region of the Cité Europe which is a commercial complex serving the Channel Tunnel, and which might be difficult for you to access on foot.

I used to enjoy a fish meal at Le Channel, but it's over 10 years since I was last there, so am not sure if it's still good.

Two other thoughts.

If you want to do a channel port, I'd strongly suggest you swap Calais for Boulogne which has a much more 'French' atmosphere, with a market in the Place Dalton, the Nausicca sea world centre and the Cathedral in the quaint old town up the top of the hill.

Or you could visit Dieppe via Newhaven (also accessible on the train from London, but with a longer crossing). Dieppe is very fishy and if you run out of things to do there, you could take the train to Rouen which is very historic.

The other thing is, if you are going as far as Calais, might you take the train all the way to Paris .....