Summer Specials in Alps and U.K. : Free bikes in Zurich, cheap travel in Northern Ireland, economy lodging in the Shetlands and York.

Budget travelers heading abroad before the end of summer can take advantage of free bikes for sightseeing in Zurich, Switzerland; economical do-it-yourself packages for traveling in Northern Ireland, and off-beat budget accommodations in the Shetland Islands and in the ancient English city of York.

Bikes can now be borrowed for free in Zurich. The program--sponsored by the Zurich daily newspaper Tages-Anzeiger, the Zurich public transportation system and a variety of private companies--will operate until Oct. 31.

One hundred bikes are available every day, 8:30 a.m. to 9:30 p.m., at depots at Paradeplatz and Bellevue--the main tram transfer stations--in central Zurich. You will be required to leave an identity card as a deposit.

YHANI, the Youth Hostel Assn. of Northern Ireland, is offering do-it-yourself packages that include transportation and accommodations for visiting Northern Ireland.

Northern Ireland’s seven youth hostels offer beds at an average rate of $8 per night. There are no age restrictions, and some have specially designed, self-contained accommodations suitable for families.

The hostels are located near key points of interest, including the cities of Belfast and Londonderry, Giant’s Causeway, the Glens of Antrim, the mountains of Mourne and Fermanagh Lakeland.

YHANI’s seven-day “Go As You Please” vacation package for Northern Ireland includes six nights’ accommodations and a seven-day transportation pass, all for $97.

The “All Ireland Rambler Packages” start at $142 (less for travelers under 18), and include seven nights’ accommodations and an eight-day transportation pass for the Republic of Ireland as well as Northern Ireland. Packages are available for 14, 21 and 28 days.

YHANI also offers activity holidays that include cycling, walking and riding.

For more information, contact YHANI at 56 Bradbury Place, Belfast BT7 1RU, Northern Ireland, United Kingdom; from the United States, telephone 011-44-232- 324733, fax 011-44-232-439699.

Free copies of the 1992 YHANI brochure, with details on the seven hostels, is available from the Northern Ireland Tourist Board, 276 Fifth Ave., Suite 500, New York 10001-4509, (212) 686-6250.

Travelers coming to Northern Ireland from Scotland can now make the trip in just 90 minutes, year-round, on the new SeaCat service that began June 1.

The SeaCat crosses five times daily between Stranraer, Scotland, and Belfast. It carries up to 80 cars and 450 passengers, and docks in Belfast near connecting train services. The one-way fare for foot passengers starts at $35.50 for adults, $27.75 for students. Reservations can be made through travel agents or by calling SeaCat: 011- 44-304-240241.

Budget travelers heading off the beaten track in Scotland can now arrange low-cost accommodations in “bods” on the Shetland Islands.

The Shetlands, a group of more than 100 islands northeast of the Scottish mainland, were settled by Norsemen in the 9th Century.

Bods are furnished cottages that have cooking facilities, toilets and cold water, but no bedding or linen. Four bods available through the Shetland Islands Tourist Board can sleep from seven to 18 people, though only one has electricity.

One bod is the former home of researcher Jonnie Williamson, who discovered an inoculation against smallpox. Another is an old fishing lodge perched on a cliff, formerly used by whalers.

To rent a whole bod costs about $23 a night; single accommodations are about $5. Book by calling 011- 44-595-3434.

In the historic city of York, about 310 miles northwest of London, students and youth groups are offered accommodations that include breakfast and dinner at the York Racecourse Centre for about $33 per person, per night.

The facilities are available year-round, except during horse-racing meetings--this year, that’s Aug. 15-Sept. 17 and Oct. 4-13.

Accommodations are in four-bedded, twin and single rooms. A cafeteria is on site, and a swimming pool and recreation center are within walking distance. Rooms must be reserved by contacting the York Racecourse Centre, 5 High Petergate, York YO1 2EN, England, telephone 011-44-904- 636553, fax 011-44-904-612815.