You plan Warsaw, Krakow and Bratislava. Logistically the best would be start with Warsaw then Krakow then Zakopane. From Zakopane there is indeed a direct bus going to Poprad. However not in ski season but from June 15th to October 15th. (At least this year). Otherwise you can still do it by bus but get off in Lysa Polana where the Polish bus ends, walk few meters across the border and catch a bus to Poprad. From Poprad there are many trains to Bratislava (about 4 hours). The direct bus goes in the main hiking season. Poland or Slovakia Tatras? Poland has 1 third of Tatra National Park, Slovakia 2 thirds. If you are a hiker and will be there in hiking season then I recommend hike up the highest mountain of Poland Rysy from the lake of Morskie Oko. Start early morning from Morskie Oko. While the hike is not for beginners you won't need any technical climbing skills just steep hike up (and then down). The hike up Rysy is little easier from the Slovak side (from Popradske pleso /lake/). If you are not a hiker stop at least at Morskie Oko. It is a gorgeous mountain lake right on the foot of Rysy. If you don't want to walk from Lysa Polana you can take a horse drawn cart toward the lake. Google "Morskie Oko" for pictures and more information. I already wrote about Slovak side of High Tatras. You can find my post under Best Destination in Slovakia. Click on country Slovakia. I mentioned few spectacular hikes there. If you would like something easier (or more difficult) you can get all necessary information at Mountain Rescue Service (Horska sluzba) in Stary Smokovec. If you don't want to hike take a cable car from Tatranska Lomnica to Skalnate Pleso and from there up Lomnicky stit (second highest peak in High Tatras). Breathtaking views. The highest peak is Gerlach. Hike up allowed only with a mountain guide, inquire at Mountain Rescue Service in Stary Smokovec.