I agree that he's an undeveloped ENFP. Or rather he's an undeveloped ENP -- MBTI theory states that the auxiliary perspective is refined around age 10-12, and Calvin fits that profile quite well -- Ti versus Fi is somewhat ambiguous, but he does seem to be developing into a Feeler.
Hobbes is probably an N, however (INTJ). He's every bit as abstract and Calvin, despite being more down to Earth, and is very good at predicting the moves of people around him. His favorite hobby appears to be fucking up Calvin's zany plans after determining how they're likely to conclude.