With the advent of summer vacation season, many tourist businesses are trying to offer travel incentives to potential customers concerned about rising fuel costs. A luxury resort in Chicago is offering a $35 gas coupon to patrons booking at least a four night stay. 19 lodgings in San Luis Obispo County in Cali are providing gas gift certificates, and even ritzy Aspen resorts are ponying up to help customers save a little dough. But Trudy Coxe, chief exec of the discount-offering Preservation Society of Newport County, R.I., likes to keep things in perspective; in regards to the current prices, she says, "Any increase is World War III in the minds of most Americans." Despite indications that costs will continue to move upward, AAA and the Travel Industry Association are still predicting an increase in driving vacations this summer.