Bob McGrath went whale watching last week off the coast of Maine. That's the kind of thing folks who work on children's television shows do.

McGrath, the super friendly "Bob" on Sesame Street, has been singing his trademark "People in Your Neighborhood," and the quizzing "One of These Things Is Not Like the Other One," to dancing muppets, jovial personalities and curious youngsters for more thaN 20 years.

And sometimes, he says, people sing them back to him.

"There's a local Mexican restaurant here," he says from his summer place on Mount Desert Island, Maine, "and I was asked to come by and say 'Hi.' When I walked in the kitchen, they started singing, 'One of These Things ...' and they did things like put the bread in with the chips and moved the guacamole to the wrong place."

Silly people.

Bob is a such happy fellow. Friendliness simply oozes from his voice. It's hard to imagine him in any other place besides "Sesame Street." The idea that he used to sing pop tunes on Perry Como's show, the Gary Moore Show and "Sing Along With Mitch" (Miller) is so, well, grown-up.

And just think, if he hadn't been strolling in front of Carnegie Hall on 57th Street one day 22 years ago, it would have never happened -- there would be NO BOB!

"I bumped into a college friend on 57th Street," recalls McGrath, "and he said, 'You should come take a look at this show. They're hiring some hosts. You interested?' I said, 'No.' But I went to check it out anyway, and saw this amazing thing they were doing. I auditioned with loads of other people -- and got it."

Now, McGrath spends several months a year taping "Sesame Street" and the rest of the time performing in concert around the country.

This week, he'll be in town hosting the 20th-annual International Children's Festival at Wolf Trap, a three-day-long extravaganza celebrating the performing arts from around the world. Featured acts include: the Halau Hula Olana dancers of Oahu, Hawaii, the National Symphonic Youth Band of Costa Rica, Incolballet of Cali, Columbia, the children's choir of San Juan, and the Tennessee Children's Dance Ensemble, featuring performances by children and adults from North, South and Central America.

There will also be workshops for children in colonial crafts, rug braiding, woodcarving, tissue collage and Oriental painting.

The festival is Saturday, Sunday and Monday, 10 a.m. to 4:30 p.m., rain or shine. Tickets are $6 for children and senior citizens, $8 for adults and teens, and are available through the Fairfax County Council of the Arts or at the Wolf Trap box office. Children under 3 will be admitted free. For information, call 642-0862.