Take a trip on the Mark Twain Riverboat. We took the one-hour sightseeing cruise that takes you on the river and points out landmarks. There's a dinner cruise or a Captain's Sunday Lunch option.
If you go:
Boarding begins 30 minutes before the scheduled departure time. Times vary depending on season and cruise choice.
Links:

Center Street Landing
Hannibal, Missouri 63401 USA
info@marktwainriverboat.com