Is it worth hitting the beach in Kenya/Tanzania, if so where?
 

I am overwhelmed in trying to figure out where to go... you guys have all given me amazing advice... My trip will probably be in September. The only thing stopping me from hitting South Africa is that my sister in law is from there, and I am thinking maybe one day I will go back there with her... This trip is on my own. That is the only reason that I am aiming towards Kenya/Tanzania... So for a 2-3 week trip, is it worth it to head to the beach for some relaxation/diving/etc?

Worth it for relaxation? No. You can relax on safari.
Worth it for diving? Not a diver, so I'll let others chime in.

Your reasoning on South Africa makes sense.

If you have 3 weeks total, then that's about 17-18 days of activities. You could do 12-14 on safari and 4-5 of water activities.

If you have 2 weeks total, then I would not reduce below about 11 days devoted to safari activities.

Zanzibar was quite a nice respite for a few nights after our safari in Tanzania.

The coastal areas/islands at Kenya or Tanzania are lovely for some R&R after safari. And, if you have a 3/week holiday, you certainly have the time to spend at either of these. With only 2/weeks, it's pushing it... something will have to go!

Remember though getting to the coast/islands will entail flights, unless
1) you take overnight train in Kenya between Nairobi/Mombasa
OR
2) in Tanzania a 10/hr bus drive to DAR and then ferry or flight to ZNZ Isl.

and then flight (or train or bus) back to your departure airport.

During September, if you are adventurous and you want rest and seclusion, you could try Pangani village in Tanzania – the timing would be right for the beach. I know it is about a six-hour drive from Arusha. If you get bored with the beaches, you can arrange safaris in Amani Reserve or Saadani National Park, or you could viist Pemba Island of Zanzibar nearby.

Sandi, do they have charter flights there yet(to Tanga)?

Khakif -

Guess if one has the money and there's an airstrip, a charter flight can be arranged to most locations.

We thought Zanzibar was definitely a worthwhile add-on after our safari time in Kenya and Tanzania. We had a 3-week trip in late September/early October that included 10 days on safari in Kenya, 7 days in Tanzania's northern safari circuit, and then 3 nights on Zanzibar. It was a beautiful and fascinating place -- not just for relaxing, but also for Swahili culture, food and music in Stonetown, visiting the red colobus monkeys and seeing the Jozani forest, an afternoon spice tour, and then a day on the beach and wandering the tidal flats, seeing the seaweed farming near Pongwe. I don't dive, so I can't offer an opinion about that, but I do wish I'd had an extra day to do some snorkeling.