Hannibal is certainly one of the major Twain pilgrimages, and
you should find plenty to keep you busy for at least a full day.
I visited in July of 1990, on my way to New York from Arizona,
but the timing was wrong to see the Clemens home. It was
undergoing a complete restoration. As others have said, Hannibal
has its carnival aspects, but the street with the Clemens house
and museum has been kept up well.
The Becky Thatcher bookstore was one of the highpoints. There
was an excellent selection of things to buy, and the people who
worked there were pleasant. Just North of town, the cave was
probably the high point of the day. On my way back to town,
there was a lookout point with a fantastic view of the town and
Hannibal in general is something of a faded rose, with many
Victorian homes that look impressive from above, but needing a
bit of work when seen from up close. There is a good book
concerning present-day Hannibal called "White Town Drowsing" by