Dobre den space virgin !
I have spent a few days in Bratislava and in the eastern part of Slovakia about 2 years ago.
I can't tell you much because I'm right now travelling in Asia, so don't have my travel books from Slovakia with me, but since you do have friends there, they should be able to guide you and give you good advice.
The capital is an interesting city, mixture of ugly communist architecture (if I may call it so !) and medieval buildings in its old part. And yes, there is good outdoor activities to do in Slovakia like trekking in the Tatras. In April, skiing is also still possible over there, weather permitting. At that time, around the High Tatras, it's splendid because the sommits are covered by snow but elsewhere is green and trees are blossoming ! Gorgeous ! Great pictures to make !
Travelling in Eastern Europe out of season will give you the possibility to bargain any hotel-guest house price ! Don't be afraid to ask for discount. For example, I stayed in one guest house for half price, only one night, upon condition I leave before 7 am, so the staff would not have to report my stay and put the money in their pocket !!! Elsewhere, I usually got a few dollars a night discount, paying around 8 $ instead of 10.
Will you stay at your friends' place in Bratislava ? If not, low budget lodging is hard to find at such time because hostels and students dormitories are not opened yet. I was lucky to be found by a lady on the street who offered me a room in her house for a few bucks a day. It wasn't super clean but a great human experience.
I have found people to be humble and very quiet, also a little distant but very polite and open when you show them interest. English is spoken in most touristic areas but german is more useful ! And if you intend to avoid the normal tourist paths, well... better bring a travel phrasebook or something with you.
So that's it for now. I hope this will be of any help to you. Have fun and enjoy Slovakia !
people are people