Yeah, depending on who you are moving around with, your weekly expenses could differ greatly.
Take fuel into account. If you have a vehicle that you will be using, then that's going to be about $40 a week for fuel. If you do not have a vehicle, but would like to rent one, then that's going to be at least $50 per day. If using local transportation, then your costs are significantly lower, even if you hire out the whole taxi to go where you need to go.
Eating out is relatively cheap if you stick to the "street vendors". Once you start going to the restaurants, that's when prices can add up. a good meal could cost about 20-45,000LE a plate(roughly 4-10 dollars a plate) Obviously there are more costly meals, but as you said you aren't going to be over extravagant.
Nightlife is relatively cheap if you are earning and spending dollars. Most bars/beach bars are free to enter and do not require any sort of minimum purchase. The clubs can range anywhere from 15,000-80,000 depending on where you go. OFFICE for example is on the higher end of the price scale, and Aces is on the lower end.
Bottled water is inexpensive and Guhvies has given your the correct prices for drinks and beer. Redbull can be pricey, so if you are a red bull fan, then I suggest bringing your own. Star beer is cheap and a National favorite, make sure you enjoy one while you sit at a beach bar somewhere in Aberdeen listening to Celine Dion (yes she is still a hit in Sl)
Must see places: First off, if you want to actually go to a beach and swim and have a good day relaxing on the beach and swimming, then I would suggest not going to lumley beach, but rather that you go to River #2 It's about 12 miles from Lumley. (you can see it on google maps. It is an amazing place. Entry with a car varies depending on who is working the gate can be about 10,000LE but you can also park elsewhere for free. Rental for a beach hut is about 30,000LE per day (can also vary depending on who is there). Be sure to take a trip up the river on one of the Canoes. Also check out Borbor beach, Laka Beach, etc.... Lugi also has a nice beach. A lot less people than in Freetown.
Apart from the beaches, There is definitely the Islands as Guhvies has already mentioned. The Chimpanzee Sanctuary is also worth a visit. Charlotte falls is also a good place to go and visit. Depending on how comfortable you are traveling around SL (not just Freetown), Then I recommend you visit BO. It's a lot nicer than you would think.
Have fun, and make some new friends while you are there. I suspect that pretty soon, there will be a huge influx of Expats returning to Sierra Leone. SL is slowly getting to a stage where you can basically live a better life there than you can in western countries, and you do not necessarily have to be rich to do so. A decent job that pays $500+ a month, would be enough to have a good life in SL. Obviously you won't be jetting back and forth to the US, but still you will surely live a good life.
Finally, since you probably want a price. I would say expect to spend around $150 per week. If you lived in SL permanently, that can be drastically reduced to about $25 a week.
