Book and go Markus. That is an order! Trust me, you wont regret. It was my first time out there. I traveled alone and had doubts and concerns before going. Just remember it is a very poor country in general and it will be a bit of a culture shock if you've not been off the beaten track before. Having said that, the tourist areas on the coast are the obvious places to head for.
Me, I stayed at the Holiday Beach Club in Senegambia. Right on the beach and a couple of minutes walk from the Strip and all it has to offer. HBC has a great pool and grounds but is only 2*. I'd say that was about right. Like others have said before, an apartment would be the way to go and its what I'm looking at for my return trip later this year. Hotels can be a bit restrictive if you plan to bring guests back. You can in HBC but it costs. Also, it is pretty obvious what you are up to!
Hooking up couldn't be easier. You will get propositioned everywhere. The beach, bars, even in the street walking around. A polite smile and a hello as an opener and often the girls will engage with you. The known haunts such as Time and Brittania are nailed on to hook up. Trust me, the girls will come to you and you will be spoilt for choice but just be aware the girls in these places are certainly pro or semi-pro. Don't go early, the girls tend to arrive 11pm or later. The good girls as I call them are the ones I prefered. Often holding down jobs but not adverse to supplementing their very low incomes. Sex seems very transactional in Gambia and I understood why when I found out my little room maid was paid just 1200d a month with just 1 day off a week...about £18!
The obvious attractions are the women, beaches and the weather. If you don't mind immersing yourself in the country there are any number of trips you can do....I did 6 different places in one day which was well worth it. I fished one day on the river which again was a fantastic experience if that's your thing. I found 7 days far to short and will go for 10 days minimum next time.