See my thread(s) about Kruger and SA, one of which is in Trip Reports, just returned from 18 days there, best time I had.
In terms of Kruger, if you stay inside in the camp, then join Sunset Drive and Morning Walk, plus one more activity on third day. Considering that each activities last 3+hours (morning walk could be 5h or more) better to have one of each for separate day and rest in between.
All these activities can be booked and paid on National Parks website - do it now, if you want to get in, usually everything is quite filled up quickly.