It occurs to me that "interesujący" should be a correct answer as well
Definitely, and it would be the most natural translation. In fact, "ciekawy" means "curious", not "interesting".
Zgadzam się! Chcę znów tam podróżować...
(I hope that is correct grammar.)
Technically it's correct, but that's unusual, we rather don't use 'travel' like that. We'd say "I want to go there again": Chcę znowu tam pojechać. (or 'polecieć' if you want to make it clear that you will fly)
Could jest be used in this sentence?
You mean "Polska to jest ciekawy kraj" or "Polska jest ciekawym krajem"?
The second is perfectly natural and accepted, the first one is rather unusual, but technically can work. It's not something we'd like to teach, but we decided to accept.
polska jest interesujem krajem???
Almost, "Polska jest interesującym krajem". That's accepted.
The basic form of the adjective here is "interesujący".