This is a very difficult question - have only been to Addo once (Dec 07 / Jan 08) but am absolutely bowled over by the Park.
So much to do - where do you start! - This is some of our experience...
Getting into the main Game Area just as the gates open and then making the Oh so important decision as to which road to take! Hoping for the best ever sightings.
Sitting at either Carols Rest, Domkrag or any waterhole and having your coffee and rusks or maybe leftover braai on sarmies for brekkie.
Sitting at the bird hide and watching a desperate weaver making a nest - ever hopeful that the missus will approve, and the next day - seeing how nervous he is when she comes to inspect.
Sitting in your tent and experiencing a thunder storm of note - thankful that you managed to braai the chops before the heavens opened up and then on Old Years Eve, sharing a glass of sparkling wine, playing a game of cards with rain, thunder and lightning all around you - and might I add, thunder that made me jump out of my camping chair!
Encountering a fellow CT Addo visitor who tells you where the lions are at... and actually seeing one of them. (either Bitterbal or Nossob - not sure).
And an even more helpful "private game drive jeep jockey" who tells you where to spot meerkat, and following his advice, you get to see them.
Having a delicious supper at the Restaurant - could not eat the Kudu Steak after having seen the beautiful Kudu so up close and personal - settled for Chicken.
Stopping at Domkrag and watching the Hyena (about 5 in total) who had stored a Kudu Kill in the reeds and then dragged it out in the early morning hours to feast - with black backed jackal eagerly milling about hoping to get a taste, pied crows also ever hopeful hovering in the wings.
Bird life - both at the campsite and on the roads in the game park.
Meeting up with fellow forumites for a braai on the hottest day ever - but still having a good time.
Wallowing in the pool - the temp was seriously over the wall - the only day that we in fact never ventured out into the Park. Same day that we had our forumite braai. (WarthogB, Nightowl, Munchkin and all the SO's)
Final Verdict - Every day in Addo brings with it its own unique promise of special memories and something you just have to experience!
Go! Visit! Enjoy!