We found our beach hotel/resort lovely. We were very disappointed by Stone Town. It was hot, poor infrastructure, limited things to do...not relaxing. One morning there would have been just right for us.
Seeing Stone Town on way to airport for return is a good idea.

Check out www.kayak.com for flight choices. I think you'll find that the Emirates flight isn't your best choice (and flies over the Atlantic, which is the shorter route, even from SEA.) The shortest duration schedule is Delta/KLM (20 hours.)

For Zanzibar, I'd recommend 2 or 3 days at the beach resort such as Pongwe or Shooting Star. Enjoy the beach and perhaps visit a nearby village. Then, tour a spice farm on your way back to Stone Town. You only need one night (if that) in ST. Take a tour of the town to learn about its history. Depending on your return flight (KLM out of DAR?), you might be able to do this without an overnight stay in ST.

Are you doing a safari in Kenya or in Tanzania? If in Kenya, you might be better off going to a beach resort in Kenya, rather than having to pay for the Tanzanian visa ($100) for Zanzibar. You'll also need a Yellow Fever innoculation if traveling from Kenya to Tanzania. If your safari is in Tanzania, fly directly to Kilimanjaro Airport (JRO) on KLM.

Thank you both for your recommendations. Looks like I was misinformed about Emirates. We are going to Kenya and Tanzania. Now I think we are starting in Tanzania then ending in Kenya (Masai Mara for the migration). I feel we need to cut out Zanzibar to save some money. If finanaces change, I feel we would stay at Shooting Star and just relax. Thank you again for your info.

Why not post what your itinerary looks like, better for us to comment.

But if starting in TNZ, fly into JRO. KLM/DL/NW is your best bet. But if you can fly Emirates (I've done this from NYC) and from Seattle believe it'll be over the Pole east, connect to NBO at DXB. If you get the early Emirates flight that arrives NBO about 3pm, you can connect to a Precision Air/PW flight, within a few hours, to JRO to commence TNZ safari.

If visiting ZNZ, you will require the Yellow Fever inoculation, just for arriving from mainland TNZ.

Also, if wishing to visit ZNZ while still technically in TNZ, you then go to KEN, do you want your 'relax' time in the middle of your safari? If not, consider your beach time on the KEN coast instead, the southern beaches.

You could also spend your relaxation time somewhere like the Rift Valley Lakes. We once spent 2 nights in the middle of our safari on an island in Lake Baringo. It was wonderfully relaxing.