Bear in mind accom is at a premium for the rally.
Dakar is interesting for a couple of days, the beach at Yoff is quiet and relaxing (not weekends) and there are great bars/restaurants in Ngor. Goree is worth a visit. Inland the scenery isn't too spectacular and so-called game reserves are a little disappointing.
Heading south it is a good road to Kaolack then sand piste (in the dry season) to 50 klicks from the border. In the rainy season it is a long and tough stretch. You know you are in Gambia as the road deteriorates dramatically and the police become more vigilant (notably the drug squad).
Gambia has really gone down the tubes in the last 15 years - Jammeh is destroying his country. The "tourist coast" is well worth avoiding entirely, though Abuko is still great value. That said, if you head east (use the NORTHERN road from Barra - best road in the country -) the people are still extremely friendly (ie: not much tourism there) and the living is cheap. Georgetown is interesting and Bassé very laid back, though a little "one-horse and dusty". The road back into Senegal, en route Tamba, is ok once clear of Gambian territory.