If you're looking for a lower-key romantic dinner, I'd go with:
Malatesta Trattoria on Christopher St- at night, lit by candles, it truly feels as though you're in Florence (and I've lived in Florence). It's the best Italian food I've had outside of Italy, and the tiramisu is phenomenal. It's also REALLY reasonably priced.
Tartine on W. 4th at W. 11th- tiny little French bistro with delicious homestyle country-French dishes. It's also BYOB so you can bring your favourite bottle of wine. Amazing pastries.
These places are both in the heart of the West Village, which in itself is the most romantic part of the city in my opinion.
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