Visiting a friend the other evening, I munched several sandwich-style peanut butter cookies. The friend, not ordinarily partial to sweet desserts, had bought several boxes at the office from a male colleague who was peddling them in behalf of his Girl Scout daughter.

Well, lots of Girl Scouts and lots of parents did such hawking, because we're now informed that 2,066,090 boxes of cookies made their way into the households and the gullets of Washingtonians. Told that figure, I set out to find out how many calories were involved, but it developed that cookie boxes come in so many configurations that computing the answer would baffle an Einstein. The figure must approach the billions.

Suffice to say, the cookie sales locally were 19 percent above the previous year, the largest increase of any Girl Scout council in the nation, according to area council President Barbara Lowis Lehmann. The earnings provided 64 percent, or $1.9 million, of the budget for supporting the 44,000 girls aged 5 through 17 in the program in the metropolitan area.

Oh, before we go along -- please pass one of those chocolate-covered mint crisps. Crediting a Key Punster

An editorial Wednesday recalled that, 20 years ago, an unnamed Washington Post reporter had described Francis Scott Key Bridge -- now in the initial stages of renovation -- as "The Car-Strangled Spanner."

The anonymous reporter deserves credit for devising that marvelous phrase. He is Willard Clopton Jr., who ultimately decided word-crafting wasn't for him and is now a practicing psychologist in Cincinnati. Detour Deterrent

The preceding item brings us to a discussion of the new 27th Street detour that directs Virginia-bound traffic on K Street NW toward Roosevelt rather than Key Bridge.

A sign at the 27th Street turnoff indicates that traffic destined on southbound Rte. 29 should take the detour.

There is just one not-minor problem: once you get onto the detour, there are no further signs posted -- by either the D.C. or Virginia highway department -- indicating what route to take to get off the detour and back onto Rte. 29. It's easy in Rosslyn -- but only if you know how. Help Them Blow Their Horn

From the Salvation Army comes an appeal: John Morrison, a new local music director, has launched an aggressive program of training kids to play in army bands.

Said Kathy Carroll, the army's local communications director, "The army gives the children free lessons and, when possible, lends them instruments for practice at home. Most of the children . . . could not afford to purchase their own instruments." However, she said, the supply of brass instruments has been exhausted.

"Desperately needed," according to Carroll, are trumpets, cornets, trombones, baritones, tubas and drums.

For more information, Morrison is available at P.O. Box 2166, Washington, D.C. 20013, telephone 783-9085; instruments may be taken to the army's community centers at 1318 Ninth St. NW, 3335 Sherman Ave. NW or 1211 G St. SE.