TODAY IS THE LAST DAY before air fares rise on some kinds of tickets. Tomorrow, major US airlines are planning to hike fares on seven-day, advance-purchase tickets carrying a 50 percent cancellation fee. The increase (about $20 per round-trip ticket) is in addition to the fuel-adjustment rise announced June 19 ($6 to $14 per round trip). AUDIO TAPE TOURS for motorists and walkers are increasing in popularity. A 1987 catalog from Comprenensive Communications lists a variety of audio tapes covering points of interest in some 15 states, nine US national parks, Canada, Great Britain, the Netherlands, France, Ireland, and Italy. Prices start at $11.95. Also listed are video tapes of other selected locations in the US and abroad. For a free copy write Comprehensive Communications Inc., PO Box 385, Scarsdale, NY 10583; or call (914) 472-5133. SUMMER IN THE CARIBBEAN offers reduced hotel prices, lower air fares, smalled crowds, and special celebrations and activities without peak-season crowds. The refreshing West Indian trade winds keep temperatures constant, averaging only five degrees higher than in the winter. For information and a calendar of events, write Caribbean Tourism Association, Dept. MC, 20 East 46th St., New York, NY 10017.