Sicily plan: 5 nights ambitious but doable?

It doesn't seem like a great idea to have such an aggressive schedule on top of 2 weeks of travel. You're going to be more tired than you think by that point, so why add a supercharged schedule at the end? That's a recipe for fatigue and travel "burnout".

I would concentrate on only one region with the time you have...meaning, I wouldn't try to cover both west and east sides of the island. You could easily spend two full days just in Palermo itself; the remaining time could be for nearby side trips (Erice, Monreale, Trapani, Marsala region, etc). You're really giving short shrift to Siracusa (not even a half day when you take transit time into account, more like a drive through) so your schedule will be a lot of driving and not much quality time spent in each place. Palermo-Siracusa-Taormina in one day is just too much (did you look at driving times?). Palermo-Monreale-Segesta all in one day is also too much. To me this is a really aggressive schedule that will likely leave you exhausted, and your time seeing each site will be really curtailed.

Too much.

One big hurdle I see is day 2-3 . Palermo to Siracusa is 3-4 hours. Then I assume you want to see Siracusa, and still arrive in Taormina for that night (2 hours?). How do you see that working? What do you want to see in Siracusa? You can't see Ortygia and the archaeological park in half a day. I love Siracusa, but if you dropped it and went straight to Taormina you'd have more time. Or skip Taormina and see Etna from Siracusa?

Also, day 1 is impossible. You can't see Palermo, Monreale, and Segesta in one day.

With 5 days I'd fly in and out of the same side.
If Palermo - do a day or two in the city, a day in Monreale and Palermo, and then Segesta and maybe Erice?
If Catania - split between Taormina and Siracusa, with Etna and maybe Catania itself.

Sicily is too big to be an "add-on." It's best to leave it until you have more than a week to spend there. There are plenty of great places to spend a few days on the mainland.

If you are firm in your decision, then take Agnes's advice and see one side of the island or the other. There are frequent buses to Monreale. Traffic is awful, parking is worse. At least on the bus, you can relax. Driving in Palermo and Catania is difficult, because of one-way streets but mainly because of the traffic congestion. Have you ever been in a car in midtown Manhattan?

Thanks folks!
OK, you've convinced me to stay East.

Do you prefer Catania or Taormina as a base?

Do you prefer Catania or Taormina as a base?

Picking a base depends entirely on how you plan to spend your 5 days and where those places are located vis-a-vis your base (assuming you want to minimize transport time). If you want to head down to Siracusa for at least a day, then Taormina is too far north for a day trip. Catania is really a well located hub, although it doesn't have the beauty or prices of Taormina. Comparing Catania to Taormina is apples to oranges - one is a big bustling city, the other a compact resort town hanging on a cliff. I stayed in Siracusa but that was because I was concentrating on all the Baroque towns in the Val di Noto/ the Hyblean mountains in the southeast.

This map may help:
http://www.bestofsicily.com/roadmap.htm

Too much. Stay in Amalfi or Croatia.

I adore Sicily, but I commend to your attention Montenegro. Just south of Dubrovnik (which I'm assuming is on your Croatian itinerary), and you can really accomplish something in 5 days there. If your add-on must come after Amalfi, rather than before, perhaps Matera and part of Puglia.

Catania has a very attractive historic district and a lively market surrounded by unexciting sprawl. Taormina is more Capri-like: expensive, extremely beautiful and utterly inundated with tourists. I preferred Siracusa to both, and then you have the Baroque towns. Five days would be a real shame.