If you like Moules frites, go to any one of the Leon's chains in Paris (there are a ton). They are awesome, I highly recommend them. (Yes, they are a chain, but their mussels are delicious).
Ripaille -- cozy little restaurant with great fish dishes
69 Rue Dames 75017 Paris
A little more expensive, but Cafe de Flore or Les Deux Magots in Saint-Germain des Pres. Go for lunch, it'll be cheaper, and the people watching is a great way to spend an afternoon (or evening).