Happy ever after in the dude ranch biz. Fun on Big Mac, Sunny, and Party-Party at Finca Ob-la-di, Ob-la-da. COSTA RICAN COWBOY

One of the more practical aspects of trail-riding through the Costa Rican countryside is that it puts about four feet of horseflesh between you and the fire ants. It also provides a nice altitude and pace from which to observe the incredible diversity of fauna and flora that hop, skip, crawl, and blossom across this country.

Andy and Avie Gingold, a couple of gringo transplants from the Upper 48, operate Finca Ob-la-di, Ob-la-da here in the outskirts of San Jos'e. (The ``gringo'' appellation is by no means a pejorative here; rather it is a term of affection used by Costa Ricans for foreigners.)

Ranch named for a Beatles song

``We got the name `Ob-la-di, Ob-la-da' from the Beatles' song about a family with two kids who live out in the country and are very happy,'' said Avie blissfully.

``It seemed to fit us exactly,'' she continued, helping us saddle up Big Mac, Party-Party, and Sunny early that misty morning.

``We're going to take our time,'' said husband Andy, as he led us three novice buckaroos out through the ranch gate. ``We're going to explore about 10 miles of trails and pastures in a comfortable, relaxed, and safe way.

``Now if you're not comfortable at any time, let me know,'' he added.

I thought for a moment about raising my hand.

This was not a fly-away ``head-'em-off-at-the-pass'' cowboy ride. It was, rather, a slow and relaxed nature tour, a sort of field trip on horseback.

Andy gave a trotting commentary, pointing out a strangler fig with a death grip on a host tree here, an iridescent blue morpho butterfly there.

Andy - thin and tough as sugar cane - is not a professional naturalist. But after 12 years here, he knows more than your average armadillo about the local terrain.

``See those trees over there,'' he said, turning and leaning over his saddle. ``We call them `naked Indians,' or `tourist trees,' because the bark is red and peeling,'' he said with a grin.

Not all groups are as enthusiastic about the wildlife as our small troupe is. ``It's amazing. There's so much life here, and some people just couldn't care less. I can usually tell after one or two explanations if the group is interested or not,'' said Andy, interrupting himself to point out a pair of toucans peering curiously down from a mimosa tree.

We walked our steady steeds up steep roads through coffee plantations, cooled our feet in mountain streams, and trotted to a small, humble farmhouse, where we were invited in and served homemade tortillas.

A fine appetizer for the lunch to come.

Dessert near a waterfall

A four-hour sit in the saddle works up a lot of sweat and a powerful appetite. An abundant Costa Rican lunch, complete with the ever-present dish of black beans and rice, was prepared for us outdoors back at the ranch. And the best was yet to come.

After lunch we were given a basket of fresh native fruit and pointed in the direction of a trail dotted with red impatiens leading into the dark, overgrown woods.

A 15-minute walk through overgrown jungle ended at a vast canyon, where a 50-foot waterfall spilled into a natural pool.

Flock of green parakeets

The canyon wall was alive with nests of chattering, redheaded green parakeets. Some 400 pairs of birds squawked and bickered as we swam in the sparkling water and gorged ourselves on fresh fruit.

Satisfied, we flopped into large, hand-made Costa Rican hammocks set along the water's edge. There we whiled away the afternoon, staring up at the occasional black vulture that soared silently in and out of the walled shelter.

Having to drag ourselves from that gently swaying hammock and sublime spot seemed the ultimate injustice.

If you go

The one-day ``Trailride Adventure'' I took costs $75. Finca Ob-la-di, Ob-la-da also offers a three-day horse outing for about $250 per person, all gear and meals included.

Bird-watching tours that start at 5:30 a.m. can be arranged. For that tour, Andy hires a professional ornithologist.

There's a certain amount of flexibility at the ranch, so custom tours may be organized to meet your specific interests.

For further information, you can write: Finca Ob-la-di, Ob-la-da, Villa Col'on, Costa Rica; phone 49-11-79, or telex 2865 ALAVAR C.R.