ALL TRAVEL BETWEEN KATHMANDU, NEPAL, AND LHASA, TIBET, has been temporarily halted because of flood damage. The Arniko Highway - the only overland link between the two cities - probably won't be repaired until the monsoons end in September. Lhasa can be reached by taking Royal Nepal Airlines to Hong Kong and then CAAC, the Chinese national airline, to Peking and then on to Lhasa. Americans in Nepal can call the US Embassy in Kathmandu (411-179 or 412-718) for information. VISITORS TO MEXICO CITY interested in a day trip to the colonial city of San Miguel de Allende should investigate El Constitutionalista, the new air-conditioned ``supertrain'' that departs daily. Write Casa Nevada, Hospicio 35, San M. de Allende, 37700, Mexico; or call (from US) 01152 987 547-3190 (Mexico City), or (212) 838-2949 (New York). AMERICANS ARE NO LONGER REQUIRED TO PROVIDE PHOTOS FOR FRENCH VISAS, though the form still has a space for one. A visitor's passport is still required by the government. Transit visas, for those passing through France, cost $3. Three-month tourist visas cost $9, and three-year tourist visas, $15. THE PUBLICATION `RUNZHEIMER REPORTS ON TRAVEL MANAGEMENT' says that business travelers can find the best corporate-rate hotel-room discounts in the US in Honolulu, San Diego, and New Orleans.