Hanoi Old Quarter has a street cafe on every corner - you pick what you want & sit on the street (with little tables). Probably best to go with a local person - they will know the best ones!
They're incrediably cheap: three of us ate our fill for £1 in total.
Failing that, try the local cafes (where you sit inside - with the kitchen at your elbow). Cost me £1 for a massive plate of stir-fry beef & Chinese cabbage (delicious) & two beers.
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