Looks like a very nice trip, sealonsf. May is the long rains in Northern Tz. As long as you're prepared for that, you'll be travelling in the green season which means lots of wildlife, with few crowds and good rates.

I have a few thoughts:
1. Tarangire is not at it's best for wildlife in May. Some animals - including the elephants for which the park is famous - leave the park during the rainy season. Having said that, I understand that the elephants are staying in the park more and more because of increased poaching outside the park! The high vegetation during the green season can make game viewing challenging, but Tarangire is one of my favourite parks just for the scenery alone.
2. I'd be inclined to fly to Zanzibar. The bus trip is 9-ish hours on Tanzania's busiest (read as scariest) highways. I've been on part of the highway - it passes some gorgeous scenery including the Pare and Usumbara Mountains.
3. You might want to stop at Olduvai on the way from Tarangire to the Serengeti. It's a longish drive depending on where the Wilderness Camp is set up in May, and you might want to stretch your legs.
4. Where are you staying on Day 11?

Others can help you out with Zanzibar. I hope to get there some day.

Have fun with the planning.

Ditto Calo's #1 comment.

Day 2: Fly Dar es Sal to Kigali, Rwanda. Overnight near Volcanoes National Park (hotel?).

If you are looking to save $$ and be closest to the ranger station--so close you can walk there, Kinigi Guesthouse is nice. I spent 5 nights there. Restaurant with good food on the premises. It's clean, friendly staff. VERY BASIC, SIMPLE rooms with bathroom/shower ensuite. Great birding, my guide who was a big birder spent time on the grounds for additional species. It is where I'd go again rather than somewhere nicer like Gorilla's Nest, where I stayed the first time.

Even though the permits are costly. I'd try for 2 gorilla visits just in case your one visit does not go as planned. What if you are ill, what if there are buffalo that prevent good viewing, downpours?

Check out the "It can happen" post.
http://www.fodors.com/community/africa-the-middle-east/it-can-happen.cfm

Day 4: Fly Kigali to Kilimanjaro. Get picked up at airport to start Safari. Overnight Ilboru Safari Lodge in Arusha.
Day 5: Arusha National Park, mixed hiking and game drive. Overnight Ilboru Safari Lodge.

Not sure on your flight times day 4, but here are 2 suggestions.
(1) If you have extra time in Kigali, go to the very moving Never Again Memorial in Kigali. Even one hour there is worthwhile. It's free.

(2) If you have a couple of hours in Arusha, you could do a drive around the park before going to your hotel.

For the mixed hiking, consider a canoe trip on Small Momella Lake for about 2 hours. Very peaceful, safe with life jackets, no waves. View wildlife on shore, a couple of distant pods of hippo families and if you want snakes sunning in trees along the shore. It was a highlight of my time in Arusha.

End of comments for now. Will respond again later.

That didn't take long.

Calo's #2 comment. I would think you'd be on the road about 7 hours from Tarangire to your Serengeti destination, depending on where the camp is. I'd ask for times/distances at least for that leg of the trip. A stop and stretch would be nice and if you do it here it won't be reducing your wildlife viewing time to incorporate Oldupai.

No Zanzibar experience.

To help decide May vs. June, find out the cutoff date when prices increase for where you are going in June. Sometimes it is June 1, 15, or 30.

Looks like a great trip!

Admittedly, a full itinerary as you're trying to get everything in, but as long as you know there will be some long days, whether flying or road transfers... it's your safari.

Agree that Tarangire is best between July-Oct, but the game is still there in May or June and the park won't disappoint. The grasses might be high some areas and small game can be difficult to spot, but not the ellees and giraffe.

Agree also that the drive from Tarangire to Serengeti will be long, easily the 7/hrs... but it is what it is. However, do arrange to stop mid-way for lunch and get out of the vehicle to stretch (Oldupai is good). Believe Wilderness Camp will be in Central Serengeti at this time.

Prices will increase as June 1st, so May the better option if budget ann issue.

Also have to ask... on Day 11 when heading to Ngorongoro, where will you be staying that night? On crater rim? (best if planning to do crater tour next morning), or outside? which will only add to the time it gets to get down on the crater floor.

Once back at Arusha, you can take a flight from here to Zanzibar, no need to head to JRO. And, depending on flight departure times out of Arusha, even coming from Eyasi, might be able to do the same day rather than overnight in Arusha.

Thanks for the great tips. Regarding Day 11 - we plan to stay at Ngorongoro Wildlife Lodge on the rim. All of these places were suggested by Base Camp, the safari operator we plan to use.

Since I wrote this up, I'm pretty sure we're going to pass on the gorilla trek/Rwanda this time around. We are flying into Lusaka and will be in Zambia for about 5 days before we go to Tanzania. With the cost of internal flights, we had to let something go. I'll check into a canoe trip. Sounds great. And a flight to Zanzibar sounds better than worrying about getting there on a bus.

Thanks again.

Wildlife on the crater rim is good.

The flight cost ARK or JRO to ZNZ is 1/hr; cost about $200/pp. Sure better than 10/hrs (mostly in dark) on a bus and then 2/hrs ferry ride.