It sounds to me like you have Barcelona all figured out. For sure do not miss La Sagrada Familia (and see other Gaudi if you can.)

I have not been to Myknonos, but hope to some day, and when I do I will take a day trip to Delos.

For Santorini, we stayed in Oia and were very, very happy there. Fira might be cheaper, not sure.
We bargained a bit with a guy and got a good deal on a cave apartment for a few days. The view of the caldera was literally breathtaking. Do not miss the volcano excursion.

For the last island, well, not sure. We had to go to Ios so that my husband could retrace his youthful footsteps. Plenty of partying and cheap accomodations there. Lots of people-watching, that’s for sure.
Prior to Santorini, we had flown to Crete, and I loved that island.
Our very first island was Corfu, which was very beautiful—there is more to that place than the Pink Palace.

I’m responding a bit late here, but:

Since you’re going to be in Barcelona for that long, I highly encourage leaving the city for a daytrip or two-day trip. Montserrat is one option. There’s also the Costa Brava beach towns north of the city (like charming Cadaqués, or Tossa de Mar with its fort and great beach; or Lloret de Mar if you’re only interested in a great beach; there’s a lot). There’s also Tarragona and Sitges south of the city. The entire Catalunya coast is lined with great beaches, a number of which are also charming architecturally-interesting towns, like Tarragona, Sitges, and Cadaqués. Sitges is very close to Barcelona, and you’ll find the old town very charming; but it’ll also be quite touristy.

Not sure why two women in their late 20s feel old. I’m barely older than you (31). While there are [sadly] some places in Greece that attract the frat crowd (college-age North Americans and Brits), Mykonos is not one of these places. Late-20s will fit right in. I can’t speak for the Paradise Beach Hotel specifically; but if it turns out to be a frat-party hostel, then by no means are you obligated to spend your time there. Just sleep and shower there, and go elsewhere to party.

The two main towns on Santorini are Oía and Firá (not to be confused wtith Thira which is an alternate name for Santorini). There’s also smaller towns, and hotels/hostels located far from Oía and Firá. My impression of Oía was: although it was gorgeous, and worth a visit, it doesn’t seem like the place for single people to stay overnight; it seemed very much like a place either for older people or for honeymooners. And Cil’s probably right; it’s probably much more expensive than Firá, although prices may have gone down as a result of the current fiscal crisis. Firá is larger than Oía, and is the party town. But not cheesy frat parties. Firá actually have quality bars; so I recommend looking for a place to stay in Firá. You can still visit Oía by bus; it’s a famous spot to watch the sunset.

Paros is very touristy, but not Cancun-ish. Naxos is quieter. But both are architecturally similar to Mykonos. Paros is also dry like Mykonos, whereas Naxos is green. So, between those two, I’d recommend Naxos, to add a quieter island, after having visited touristy Mykonos and Santorini. But also consider (and do some research on) Andros and Syros.

If you’re able and willing to leave the Cyclades, consider Crete, Corfu, Rhodes, Zakynthos, or Kos.

From Barcelona, you can take a train to Figueres, see the Dali Museum, then a bus to Cadaques to visit his well preserved house. I’m not a fan of Dali, but that was a very interesting day-trip.

They also have tours covering that route, if you think you would be more comfortable with a guide.