If you are on a budget, forget Denmark. You can spend as much per day in Denmark as you will spend for 3 days in Slovakia, Slovenia or Croatia when it comes to hotel, transportation and food costs
Croatia and Slovenia are beautiful. If you want beaches and Roman ruins, go to Croatia. If you want culture and nature go to Slovenia
Bratislava is a nice city but a week is a long time there. I haven't visited the rest of Slovakia so hopefully someone from there can answer what else there is to see
They are all different places that offer different things.
Denmark - no beautiful nature, flat. Go there if you really want to see Copenhagen, which is nice.
Croatia - boring Zagreb, Plitvice, nice sea and villages, mostly rocky and paved beaches, crowded in the summer, expensive in the well visited places
Slovenia - beautiful nature, great caves, lovely but small Ljubljana, good food and wine, friendly people, very small coast
Slovakia - boring Bratislava, some mountains but nothing like in Slovenia
I'd rule out Slovakia. Nothing special at all over there.
So you get to chose between Copenhagen, nature (lakes, mountains, caves) and sea. It's your choice.
If you don't like expensive places, don't go to Denmark!
Slovenia and Croatia form a good combo, if you like countries that are not (yet) ruined by mass tourism, but which are full of history, very pittoresque, relatively clean and relatively cheap, yet still modern enough for tourists to visit and stay. Both being relatively small countries, one can visit both within one week, especially if one can rent a car or travel otherwise.
Croatia
I stayed 3 days in Bratislava, Slovakia. It was one of my favorite places! We stayed in a nice "botel" (hotel that used to be a boat). The area was safe, known for the safest tap water in the region! And it was only a 55 minute drive to Vienna, Austria! (which did not require any border crossing, thankfully). So I'd pick Slovakia :)
