I was in Kenya and Tanzania three summers ago. You will have some difficulty finding good beer there (in the remarkable sence) but you will be able to try a myriad of different international lagers that will vary quite a bit in the finish.
The lagers are generally fine and at least usually better than the cheapened crap the macros make here (I think they use more malt) but they definitely won't blow your mind.
The best lagers are brewed in the countries which were once colonized by the germans (Namibia for example) - look for these. They are still mostly brewed by the german law of purity. Windhoek for example was a very fine surprise for me while travelling.
If you go on safari in the Ngorongoro Crater, I recommend you go to the three star lodge rather than the five star (Serena). Not only was it more fun (good entertainment - more sociable clientel, better lookout on game etc. etc.... and cheaper but the beer selection was about as good as I saw it anywhere in East Africa. I recall drinking weisbier (Frankensteiner) and a host of other german gems there.
As Jer. said, Guinness Extra Stout is available pretty much everywhere and usually below a buck. Problem is that you won't always feel like drinking it due to the extremely hot weather you will most certainly encounter - especially in Dar Es Salam.
If you want to climb one of the East African peaks, I suggest you try Mt. Kenya as oposed to Mt. Kilimanjaro. My wife and I had the entire National park to ourselves, pretty much the same scenery (maybe even a litle better since the peak is much more dramatic. It is only a couple thousand feet less than Kili. (and therefore a little less problematic in terms of altitude associated problems)...