How many are "we" ... is it a couple, or 2 friends? We'd just be doing guesswork without more key info, ie:
EXACT DATES and TRIP LENGTH -- When you say "in September" this is too vague. Many places drop hotel rates mid-september so it matters. Also 7 nights ... is that maybe only 6 days?Tell us Exact DATE and TIME of DAY you arrive ATH airport, and Exact DATE and TIME of DAY your homebound plane departs. (NOTE: don't count arrival day unless you arrive before 10 am, and don't count departure day at all.) if you are coming to Greece from another European Country, or leaving Greece to visit another place in Europe, tell us that too. All this helps us give you better advice
CHOICES -- Are you choosing Santorini & Mykonos because they are the only ones you've ever heard of? Or because your main interest is late-night clubbing?? They are the 2 MOST touricized islands in all of Greece and twice as expensive as most other islands. I can understand that Santorini is a must-see for first timers, because of the Famous Caldera View ... but Mykonos may not live up to the hype. On these isles, $50/double is a challenge! Also you don't really tell us interests -- nightlife? Are beaches important? If so, cut a day off Santorini (its beaches are black grit), and add the day to Island #2). Is Mykonos a "must" or could you do another island that's lively in September, like Paros, that would be quicker to reach from Santorini and half as expensive?
TRAVEL -- As the old saying goes "You either spend time or money" -- and since u don't have much of either, this is a challenge!
if you really just have 7 days flat, doing ferries can eat up whole days. It could take you 8-9 hours (and $60-70) to take a ferry to Santorini... vs 45 minutes by plane. But -- Depending on DATES -- plane could cost either $40 or $250!! See what I mean?
Share more -- lots of specifics; how many/ age range/ what's of interest/ what's NOT of interest -- & we'll try to help