Sofia's ok for maybe a day or two but nothing particularly special. Watch out for stray dogs, and be wary that if you shake your head at a skanky prostitute it actually means 'yes'. Some of the beer's pretty decent mind.
Skopje is a cool place, but if you want even better take a 3 hour bus ride from there and stay at Lake Ohrid. It can be a little touristy around there, though it's mostly Macedonian tourists who go, but they're very appreciative of foreign visitors. You'll find everyone says 'zdravo' to each other in the street and then they'll try to strike up conversation with you when they realise you're not from round there. Everything's rather cheap as well.
Not sure what it'd be like at easter but I went to the above places in Sept 2009 and it was mid-30s celsius.