If you are staying any length of time in Nelspruit, try the Nelspruit Botanical Gardens. They have a rainforest (artificial rain), a river, an aloe garden, a lot of indigenous plants and the birding can be quite good there. I agree, take your Sigma 50 - 500 and a monopod, but take note of what GiGi said, and do not flash your gear around when on your own in an unprotected area. If you look on the SANParks Forum they have a birding section :
http://www.sanparks.org/forums/viewforum.php?f=46&sid=7a839d021e0480c21366d022cf843e38
you might get some good tips on best birding areas in Kruger. The greatest number of birds are usually in the camps and near water such as rivers or waterholes. With regard to Mozzies, wear long-sleeves and long trousers in the evenings if you sit outside, take citronella candles and use citronella soap. I don't like stuff like Deet so I use citronella oil. You can buy something called Mosquito Wipes which are impregnated with citronella in some places, we found these wonderful - I think Outdoor and Camping Centres may have them. It is essential that you take prophylactics, starting at least four days before you enter the malarial area (this includes Nelspruit) and ending nine days after you have left the area. If you feel as though you are getting a cold/headachey/achey/flu-ish afterwards, go immediately and get a blood test for malaria. It is much easier to treat if caught early. Hope you will have an amazing time. Do you know which camps you will be in in Kruger?
Good information, but with regard to the taking of prophylactics best leave it to the doctor to prescribe as not all are taken 4 days before and 9 days after. Just ask your GP.