EUROPABUS, THE MOTOR COACH SYSTEM OF EUROPE'S RAILWAYS, now offers a year-round schedule on almost half of its 48 bus tours. The expansion of these two- to 14-day tours will prove economical to travelers because of lower off-season rates throughout Europe. Write to Europabus, PO Box 403, Old Greenwich, CT 06870. VISITORS TO SANTE FE, N.M., can experience the Third Palace Mountain Man Rendezvous and Buffalo Roast at the Palace of the Governors Aug. 13-15. The celebration, sponsored by the Museum of New Mexico, features events depicting the life style of mountain men in music, crafts, and other shows. All proceeds benefit the 18th-Century Segesser Hide Paintings recently acquired by the museum. For details, call (505) 827-6474 or 800-338-6877.