Restful means that there is some kind of relaxation and often imply that there is restoration going on. Example: I had restful sleep last night, because I had REM sleep.
Easy means that it does not require a lot of work or effort. Example: The test was easy; a five year old kid could ace it.
They are both adjectives so the way you used them was okay. But because they don't have the same definition, I would say you cannot use them interchangeably.