Watching television is the nation's favorite evening pastime, but its popularity is dwindling, according to the Gallup Poll. In Gallup's latest survey, 33% said TV was their favorite way to spend an evening, down sharply from 46% in similar polls in 1974 and 1966. Tied for second place were reading and relaxing, each named by 14%. Other favorites: staying home with the family, 13%; dining out, 10%; visiting with friends, 8%; movies and theater, 6%; cards and other non-athletic games, 4%; dancing, listening to music, doing chores and sewing, 3% each. At the bottom were drinking, athletics, church activities and fishing, 2%.