The airport is on the north end of the peninsula, on which Dakar sits at the southern end. A cab downtown would prolly take no more than 30 mins, but it really depends on traffic. I understand traffic is a mess there right now because they are redoing some of the roads. You can wander around downtown...theres several markets, can go to Goree Island, or go sit on a beach on the western coast of the peninsula. PM me if you have any specific questions, I studied there for 5 months in 2006.