Malawi is such a diverse country, especially when you consider is small size. the best part depends upon what you prefer.
You can not visit Malawi with visiting The lake, it's superb, fresh water, plenty of varying places to stay, some quiet, some areas busy with budget travellers and there is all qualities of accommodation available along the length of the lakeshore. There are several places where you can do watersports and scuba diving
There the mountains such as Mulanje Mountain
(click for photos of Mulanje)and Zomba Plateau
(Click for photos of Zomba Plateau) are both in the Southern part of the country, both great from hiking and in the North there the Nyika Plateau, rolling hills where you can do walking safaris.
Then there's the game reserves, which can not be compared to Kenya and Tanzania, but I think they provide a more adventurous experience where you have to search for the wildlife. My favourite is Liwonde National Park
Click for photos of Liwonde but there are many others
click for information on Malawi National Parks
It's a easy country to self drive around, although vehicle hire can be a little expensive and you need to be aware that in the rainy seasons some of the roads may not be passable, especially in the National Parks.
You will have many opportunities to meet local people and learn more about their culture - most people do speak English.
It's a great country...