I would say most (not quite all, but most) places Ryanair fly to are worth it for a weekend. I have been to 30 or 40 of their destinations by now and had a great time at each. There are all the obvious ones like Amsterdam, Prague, Kraków, Munich, Berlin, Barcelona and the like... Here are a few you might not have considered yet:
* Anywhere in Scandinavia will be beautiful with all the snow - hotels and food could be pricey though.
* In Germany, I find Bremen to be a great city. You can hop over to Hamburg from there as well (1 hour on the train) but honestly, I think Bremen is much cooler itself.
* Further East, I very highly recommend Vilnius. Amazing place, beautiful old buildings and very educated people. Everyone speaks great English, everything is in perfect nick. You could never, ever tell this used to be the Soviet Union.
* Gdańsk is a great city in Poland, also has a very cool old town and should look beautiful in February.
* A weekend road-trip I did before was this: Fly to Frankfurt-Hahn, hire a car, drive to Trier (less than 1 hour away). This city has lots of ruins from Roman times to offer. Then, drive on to Luxembourg (1 hour). Luxembourg is charming and there is a lot to see. Either stay here for the rest of the weekend and then drive back to Frankfurt-Hahn for your return flight (1.5 hours) or explore Luxembourg country a bit (as in, leave Luxembourg city behind and see the countryside).
Really, you cannot go wrong with most Ryanair destinations. Just look for cheap flights, Google the potential destinations and you will find there is lots to do at every single one.