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.