Senegal surrounds Gambia and there are excursions to Fathala Reserve, just north of the border, for example.
Guide for getting from Banjul to Dakar:
Get up early, you want to catch the first ferry at 07:00 as it is the only one which is not very busy and full of pickpockets. Leave your hotel at 06:30 if in Banjul or 06:00if in Serrakunda area, take yellow taxi. Ask for "Barra Ferry Terminal", go in, buy your ticket for GMD10 and then wait inside. Watch your bags. Board the ferry and cross. When you arrive, ask for a taxi to the Gambia border if he asks if you want him to take you to Senegal border say no and refuse to pay more than GMD200. Go to Gambian customs, you will be asked questions, answer honestly. After exit stamp change your dalasis to CFA francs as this is difficult in Senegal, there is a Bureau next to customs office which has better rates than the street guys. Also change pounds sterling as they are difficult to get rid of north of Gambia. Euros and US dollars are OK. Do not give to begging children until you have done all this, otherwise you will be mobbed by kids. Walk 50m to the Senegal border, get the stamp. This is easy, no questions. Take a moto taxi from the border to the gare routier bus station, do not pay more than XOF200 and ask them to take you to the Dakar sept-places seven seat bush taxi. Taxis go via Kaolack, if you are prompt you can make it all the way to Dakar in a day before darkness, however you may want to spend one night in Kaolack to get used to the differences between Senegal and Gambia before taking on the big beast of Dakar. Make sure you know where you are going to stay in Dakar, if you ask taxi driver for a hotel they will take you somewhere very expensive.
If doing the reverse journey and you get stuck at the border town of Karang/Amdallai for the night, the best option would be to spend the night on the Gambian side. There is a British run NGO called Helping Charity (http://helpingcharity.org...) which has a guesthouse there - with some very nice accommodation. It is less than 2km from the border on the Gambian side and is highly recommended as place to stop for the night or even to spend a day or two as there is a local school on site, also. Paid GMD400 per person, per night.