Another must do when you visit Hanoi, Vietnam is Tam coc. Many travel agency's arrange a trip in combination with Hao Lu (look at my first post). Most trips will go for a bike ride through the rice fields 
where you will have, as you can see, a beautiful view on the huge limestone mountians. After that they will arrange a small boat trip for you of around 45 minutes on which you will sail through 3 awesome caves (Tam coc means 3 caves in old Vietnamese). Very fascinating!
(Me in the front of the small boat)