THE CAPE COD & HYANNIS RAILROAD has begun excursions on Cape Cod for summer and fall. Trains depart from the Hyannis depot at sunset for a four-hour dining adventure with a four-course meal and live entertainment in a 1912 Victorian dining car. Write the Cape Cod & Hyannis Railroad, 252 Main St., Hyannis, MA 02801, or call (617) 771-7008. THAT LOOSE FOREIGN CHANGE THAT YOU GET STUCK WITH after traveling abroad is being put to good use by Virgin Atlantic Airways. They invite passengers to drop leftover change into envelopes found in the in-flight magazine to the United Nations Children's Fund. Virgin estimates that in one year collections for UNICEF could yield as much as $500,000. WASHINGTON, D.C., HOTELS OFFER EXTENDED BARGAIN RATES FOR THE SUMMER. Over 25 Washington hotels have extended their low weekend rates to every day of the week. These rates start as low as $19.50 per person and will be offered through Sept. 7. For a listing, write Washington, D.C., Convention & Visitors Association, 1575 Eye St., NW (Dept. S/87), Washington, DC 20005, or phone (202) 789-7000.