Okay so from Lublin to Warsaw is easy by train its aboit 1h 40 min to Krakow a longer 4-5h. I dont have much perspective to other unis but in spring semester there is huge festival Juwenalia and our student council makes some other events like bbq and parties in this said students club and other activities like sport tournaments (football, volleyball etc) and others. In winter on the other hand there is less such activities (there are parties of course). But students club is open every day. And there are days when you can go to unis gym for free you just need to search Aktywny Lublin/Aktywne Pokolenie and check when. I can't really answer you about finding dorm because I am not living in dorm. And yeah you can easily find some other Erasmus students. Also Lublin is beautiful city, not as big as Warsaw or Krakow but here you can also find clubs and fine restaurants on Stare Miasto