Lets say I want to do Santorini for a few days, what other countries/places would you combine with this trip? I have 2 weeks.
We found Santorini to be a beautiful place, but it's a pretty small island @ 35 square miles. We'd enjoy being there no longer than a long weekend.
We especially enjoyed Crete and Malta on a cruise Spring, 2016 as they're larger islands
If this is a trip from the US or Canada, I'd spend all the time in Greece since it is a bit costly to reach and has many worthwhile sights. You'll probably find you need to travel through Athens at the beginning and end of your trip; it's the most common gateway to the islands. I don't find Athens a particularly attractive city. It's mostly modern. But in addition to the Acropolis there are good museums, so it's worth a few days.
Although I haven't been to Nafplion (which everyone seems to love), I really enjoyed the eastern Peloponnese as a change from tourist-swamped islands and recommend that area highly. With two weeks, you have time to do something beyond Athens and a bunch of islands. Mainland Greece is a different experience from the islands.
Among the islands, my preference is for those that are not wall-to-wall tourists. I suggest checking out Crete (and see the interior, not just the coast) and Naxos. You could also consider one of the smaller, lightly visited islands, if you are intrigued by one that's easily reachable from another island you're visiting. What is practical in Greece sometimes depends on ferry schedules.
It's important to allow plenty of time to get back to Athens the day before your flight home. Don't schedule yourself too tightly if your last stop is on an island! Ferries and even flights can get canceled. In fact, it would be best to plan most of your Athens time at the end of the trip so there's no risk of missing your flight.
When are you planning this trip?
If its summer May through September then I would fully endorse acraven comments. If its in winter then there are other options you might want to consider.
Perhaps if you gave us some information about what you want from a holiday we can be more precise in our responses.
Ios and Mykonos are close and Interesting!
I'd visit at least one other island as well as Athens and other places of interest on the mainland (I had 9 nights).
I agree with acraven, spend the whole time in Greece. There are many choices of islands and mainland sights. Athens is well worth a few days. Delphi is worth an overnight stop. Nafplio and surroundings are good for several days. Crete has a lot to see. Many on this board like Naxos (I haven't been).