The best choice I think is wandering the around old quarter at your own speed if you do not feel up to a full Foody tour because 2 to 3 hours is not enough.
Seriously, wandering Ma may street, and others, offers a whole new world of delectable delights. Sit on the footpath and have some beer on Ta Hien St, beef always make the amazing Bun cha after beer is wonderful. You can watch Pho being created in the cauldrons on the street as well.