you can only put ONE verb after "can". I sentence 1, the thing that you "can" do is "swim". In sentence 2 the thing you "can" do is "enjoy". What do you enjoy? You don't enjoy cake or relaxation: you enjoy "swimming". The verb "enjoy" is designed to have a noun like "cake" after it. If you want to put a verb after it, you need to add -ing to turn it into a "gerund", which is a verb that is acting like a noun.

The modals are not used in continuous form...Ok