I found Dakar, by African standards, to be a pleasant, safe and relatively un-chaotic city. However, it is never going to be a main feature on any tourist brochures. My marvellous pics are here
You will easily be able to keep yourself entertained for one day, though. I would spend the morning and afternoon at Ile de Gorée which is a smashing little excursion and now geared up with the amenities for the increasing number of (mainly French) tourists who flock there. The tourists flock there for good reason - Gorée is beautifully scenic and historic, too. You can relax on a beach as well if that's your bag.
Back on the mainland, I would then opt for either the IFAN Museum or go around one of the markets. The hustlers at the markets are harmless, but an irritating and incredibly offensive menace.
Evening time, I s'pose you could go to Youssou N'Dour's bar, Club Thiossane. People rave about this place and it's a bit like the Eiffel Tower in that you're kind of obligated to go there since you're in Dakar. Personally, I was totally under-whelmed by Club Thiossane, but other people genuinely think it's fantastic.