There are some good winter beaches in KwaZulu Natal on the northern Indian Ocean coast in South Africa, a day's drive (or less) from all the safari destinations in KZN or the Kruger complex inland. The water will be warm enough for swimming or scuba diving, sea turtles in quantity, and the next day you can be seeing the "big five" in various national parks or game reserves.

http://www.zulu.org.za/index.php?districthome+24+4

Yes the Kenya/Zanzibar combination is quite common.

Here is an example. http://www.secretescapes.com/kenyan-...OrZpaT-_Rz4qRA

I'm sure there are others for any length of time available.

Iowa_Redhead may have hit on the perfect solution.

We did both....Southern Tanzania (fewer people) and diving/snorkeling on Mafia Island off the coast. It was a wonderful holiday. Here is my trip report:

http://www.fodors.com/community/afri...and-selous.cfm

http://www.fodors.com/community/afri...fia-island.cfm

Photos from our trip at:

www.tonna.zenfolio.com

Simple adivce: western Uganda. You have the range of the Rwenzori mountains, directly bordering the savannah of Queen Elizabeth National Park to Bwindi Impenetrable Rain Forest and the mountain gorillas. Just south of there, you will find the Virunga volcano range with partly active volcanoes - including Mt. Nyiragongo - the volcano with the world's largest lava lake. I took a trip with www.instinctsafaris.com a while ago and i was impressed with the cultural and scenic variety in the region between Rwanda, Uganda, and DR Congo.

Cheers

I've thought of another answer for the ubiquitous 'once in a lifetime' question.

Go where you will not want to go ever again in your lifetime.