Precision Air recently started up a red-eye flight between JNB and DAR - I think they fly 4-5 times a week. I'm not sure of the cost - check their website. (But you might want to use a travel agent to book - I know people who have had problems with purchasing via the Precision website.)

There's also a low cost carrier from South Africa that flies from JNB to ZNZ - I think it's 1time airlines (or something like that). I've never flown them, but it might be worth checking out. I doubt they fly everyday, though, so you may need to be flexible.

Or go to South Africa, Mozambique -- or elsewhere (Vic Falls?).

If you do go to ZNZ, note that the Sauti za Busara music festival is in February, and so a) this might (or might not) be something you want to go to, and b) Stone Town accommodation is usually fully booked at this time.