If you love coffee and are in Hanoi, a stop at Giang Cafe (often called Cafe Giang too) is a must.
The small cafe is two floors, filled with tiny tables and tiny chairs. The menu has several drink items, but they are most famous for their egg coffee (hot or cold). The little espresso-sized cup is filled with a shot of strong Vietnamese coffee and then topped with a sweet egg foam. Both the hot and cold versions are delicious, so choose whichever you feel most in the mood for.
Sit on the second floor for some natural light and plant life. Don't be afraid to flag down a server if you think they forgot your drink. For the most part though, the workers seem pretty on top of who ordered what, even though nothing is written down and they take multiple orders at a time.