Timbuktu is not a food destination. Options are limited and simple, but you will eat well enough. Brochettes from street stalls are cheap and reliable. For sit-down meals, expect basic West African and French-influenced cooking.
Restaurant du Nord is the reliable standby -- bread, coffee, tea, and simple meals. Poulet d'Or has slightly better food at a slightly higher price. For a cold beer, try Bar Thierry-Sabine on the main street.