I have a certain amount of expertise here because I live fairly close to Stansted and, while I've not taken a proper 'holiday' for about a quarter of a century, I love taking day trips abroad (from Stansted, Luton or via Eurostar). Further, I've spent lots of time over the past few weeks researching the best possibilities for day trips from Stansted, with the two obvious requirements being that:
(a) there has to be an early flight out and a late flight back (otherwise it's not worth going) ; and
(b) the prices have to be reasonable!
Unfortunately many of the places I've visited in the past no longer seem to have flights at the right times (or, if they do, they're mega-expensive!). However I can definitely recommend Amsterdam. The city centre is compact enough to allow you to explore many of the main sights within a day. Further, it's only a 15-minute hop on the train into the city centre. There's an 0700 flight out (arriving at 0905), with a return flight at 2130 (arriving at 2130). Fares are currently around £68 return.
http://www.iamsterdam.com/en/visiting
and
http://www.easyjet.com/en/
Mainz is another possibility. It's got the loveliest market I know of anywhere in Europe. It's also a very attractive city. You'd need to take the 0730 flight from Stansted to Hahn (arriving at 0955). Walk out of the airport and turn right to walk the few minutes to the bus station. From there, the 1030 direct bus gets you to Mainz at 1140. The 1920 bus back (NB: Wednesdays, Thursdays & Fridays ONLY!) gets you to the airport at 2030, well in time to grab a bite to eat before catching the 2235 return flight (arriving 2255). Return flights are currently priced at around £40.
http://www.touristik-mainz.de/stadtportrait.html?&L=1
Flights:
https://www.ryanair.com/gb/en/
Bus timetable:
http://hahn.orn-online.de/busfahrplan_mainz.htm
Somewhere I've yet to get to (but is widely recommended) is Berlin. Ryanair flies there from Stansted at 0630 (arriving 0930) and 0750 (arriving 1040). Trains into the city centre run every half hour, taking 30 minutes for the journey. There's a return flight at 2255 (arriving 2325). Return fares are currently around £56.
http://www.visitberlin.de/en