That’s not a whole lot of time, but I like your choices.
Mykonos is closest, and has vibrant night life as well as sight-seeing. It is from Myknonos that you can visit the famous antiquity, Delos.
But you can fly to Santorini, where you can look out upon the amazing caldera, go on a volcano hike, enjoy a mudbath, and travel to an ancient town on a donkey.
Ios is just one big party but is not without charm.
Unless you can fly, I’d probably just pick one island, probably Mykonos.
Others may offer different opinions. I know there are high speed ferries, but I’d hate to miss that July 28 departure.

I’ve been called a dick a few times here on eurotrip, so here it goes…

Come on …did you really do “research” and happen to come up with Athens + the only 3 islands that North Americans can name?

Anyways, Cil is 100% correct. Make it Athens + 1 Cycladic Island. You may find it more appealing for 20-year-olds, rather than 25-34 year-olds (However, Cil has found a quieter side of the island for 50+ folks). Plus, Ios does not have an airport, in case you’re in a hurry and need to fly. Mykonos and Santorini are accessible by both airport and ferry, and ferries include both conventional ferries, as well as high-speed ferries.

On a subjective note, I would go with Santorini, due to being everything you’d want from a Cycladic Island…it’s got the Cycladic architecture and, unlike Mykonos or Ios, it’s got the dramatic natural setting, with the towns of Firá and Oía on a cliff facing the volcanic island down below. There’s less nightlife than Mykonos, but I found Santorini’s nightlife (specifically in the main town of Firá) perfectly adequate in volume, and really good in quality. Again, I’m talking about Firá, not Perissa (which is out of the way for nighttime anyways).

Happy travels.