HONGKONG'S Mandarin Oriental plans to convert some of the floors in its hotels in Jakarta, Manila and Singapore into executive areas. This concept is followed by many hotel groups, but Mandarin Oriental will be the first top-level group in Asia to add them. In most cases, the main visible difference is the lounge on the executive floor level, which serves breakfast and cocktails in something of a ''club'' atmosphere. Food and beverages in these outlets is usually included in the cost of the room, which can be 20 per cent more than the regular tariffs. The lounges sometimes also operate as check-in areas for guests on those floors. Other benefits detailed by Mandarin Oriental are safes in rooms, coffee-and tea-making facilities, free pressing, and overnight laundry and dry-cleaning with no surcharge. For more information, contact Mandarin Oriental reservations, phone 881-1288. THE 100-room Observatory hotel opened last month in Kent Street in Sydney. It is managed by Orient Express Hotels, part of the company which runs the luxury train of the same name. The opening rate of A$220 (HK$1,200) is due to run until the end of April. This is higher than most rates for the top hotels in the city. Contact the hotel direct on (61-2) 256-2222, or fax (61-2) 256-2233. A RECENTLY released review of Hongkong hotels shows American Express steadily losing its commanding credit card market share to Visa. At its peak, in 1989, American Express had 46.3 per cent of the total, with Visa trailing with 31.4 per cent. By 1991 - the latest figures available - Visa was still behind, with 34.3 per cent, but it was closing on the 43.3 per cent of American Express. Figures from Horwath Asia Pacific. CATHAY Pacific is due to start extra flights into Taiwan today - to both Taipei and Kaohsiung. The extra flights leave Hongkong on Tuesdays, Wednesdays, Fridays and Sundays at 6.50 pm, and return from Taipei the following days at 9.10 am. To Kaohsiung, the extra flights leave at 6.50 pm on Mondays, Wednesdays and Thursdays, and return the following day at 10.10 am. Contact Cathay Pacific reservations on 747-1888. THE ANA Hotel in San Francisco is running a special room-only offer until the end of April at US$105 (HK$815). The hotel, which was previously named the Meridien, also has package offers for Friday, Saturday and Sunday nights, although these (such as one for US$128 including breakfast for two) are not such good buys. Contact ANA Hotels reservations on 845-4280. THE 18-hole golf resort at the EuroDisney complex outside Paris has opened. The cost of a round is 270 francs (HK$375) on weekdays and 385 francs at weekends; both these prices include the use of an electric golf cart. These rates run until April 30, then increase to 450 francs daily until the end of October. Rental of a set of clubs costs 100 francs and rental of shoes 55 francs. Book through travel agencies. ANOTHER London-plus-car offer has been announced. This one costs $8,800 in the low season (April 22-June 21, and from the start of October). In the peak season the price is $10,280. The cost includes round-trip economy air ticket, but only three days' car rental; airline is British Airways and the car rental company is Hertz. Contact Thomas Cook Travel on 545-4399, or fax 545-7477. Information compiled by Travel Database, GPO Box 12761, Hongkong. Fax 507-4620.