Norwegian in Oslo checking in here - avoid Oslo. It’s not a good, or even decent, place for tourists in spring. About the only thing it’s got going for it is a high density of museums and landmarks. But unless you’re travelling with someone, or meeting someone, you won’t really get close enough to the people there to really kick it off. (Norwegians in Norway are pretty stiff, particularly in Oslo)
If you do want to go to Norway, I’d recommend Stavanger or Tromsø. Stavanger is on the south-west coast, it’s homey and comfortable, the people are nice, the weather is good, it’s got absolutely gorgeous fjords (and Preikestolen is just nearby) and there’s even a good night scene.
Tromsø is way up north, if you want the proper Norwegian experience with wild and beautiful nature close by, yet still urban enough to be convenient and accessible. The people are friendly, outgoing and unpretentious and the weather in April is usually pretty good.
(If you ever do stop by Oslo, though, shoot me a PM. Might make an occasion for us Oslo Dopers to get our fingers out of our arses and have a proper Oslo DopeFest! I’ll buy you a beer, at any rate :D)
Budapest in Hungary is an absolutely lovely place to be in spring and I can’t recommend it highly enough. Second on my list is probably Dublin, then Vilnius in Lithuania. (Vilnius has architechture and beer to die for.)
A pit stop in Amsterdam is pretty mandatory, though best enjoyed if you’re travelling with friends (or hook up with other backpackers.)
A tip though is, when you’ve decided on which countries you want to visit, to check when there’s festivals in or around the places where you’re going. While April isn’t high season for it, there’s still some pretty cool scene festivals happening in, say, Rotterdam and Leipzig in Germany.