Café Rimbaud is one of the better-known eating spots in the old city, trading on Harar's most famous European resident while serving a straightforward menu of Ethiopian and Western dishes. It sits in a reasonably atmospheric old building not far from the house museum.
For breakfast or lunch it is a reliable choice: decent coffee, injera with various accompaniments, and the kind of menu that makes sense to a traveler who hasn't yet adjusted to the local schedule. It is also the kind of place where you will meet other foreign visitors and can pick up current advice about guides, onward transport, and what's worth seeing.