GETTING THERE: The closest major airport to Hannibal is St. Louis, about 100 miles to the south. This summer, TWA offers a 30-day advance-purchase round-trip fare from Washington of $158 for travel on Tuesday and Wednesday. For travel on other days, the fare is $178. A seven-day advance-purchase ticket is $271 for travel on Tuesday and Wednesday, $316 other days. For all these fares, travelers must stay over at least one Saturday.

WHERE TO STAY: Hannibal offers a good selection of accommodations, from motor inns to bed-and-breakfast homes. There are also several campers' parks and campgrounds.

WHERE TO EAT: Many restaurants feature family-style home cooking. The expected fast-food chains are there for those who prefer them. INFORMATION: For a listing of restaurants, shopping facilities and accommodations, as well as information about Hannibal's many tourist attractions (including schedules for the riverboat Mark Twain, the outdoor drama, the Mark Twain Cave and the Twainland Express Sightseeing Tours), call or write for the 1985 Hannibal Visitors Guide, Hannibal Visitors and Convention Bureau, P.O. Box 624, Hannibal, Mo. 63401, (314) 221-2477. For a schedule of the Sesquicentennial activities and special events, write or call the Mark Twain Sesquicentennial Commission, P.O. Box 1985, Hannibal, Mo. 63401, (314) 221-0114.