Nightlife? Here in Le Kef? Good luck finding any! All there seems to be are a few dead end bars which seem to close early. Your only "real" option is to try the Hotel Sicca Veneria (where I spent the night) where there's a bar area with lots of large wooden tables and quite a few drunken locals watching TV. Warning, I wouldn't go in here if I was a single woman, if you know what I mean.
Nightlife in Le Kef, like in much of Tunisia, revolves around the many cafes. The nicest during the daytime is definitely Cafe Sidi Bou Makhlouf, by the mosque of the same name, high up in the old town, and at night it also a very nice place to sip coffee or smoke a chicha. You can watch the sunset, then move inside (if it is cold...it was in March) and sit on one of the ancient benches and listen to tapes of Algerian rai stars.
Down on Avenue Bourguiba, people walk up and down the road, admiring the views, eating sandwiches and sitting one the wall cracking open nuts and seeds. People are friendly here, and foreigners are still relatively rare, so there's a very good chance someone will come and talk to you. Many tourists are wary of talking to Tunisians who approach them, and while there may be cause for concern in some beach resorts where ulterior motives may be involved (money, sex, visas...), the rest of the country, le Kef included, is full of hospitable people who want to get to know you as a person, nothing more.