Suddenly spotted everywhere: Battersea boxes. (Nancy Reagan flaunts a cluster of the little copper-and-enamel bijoux on an inlaid octagonal table in the First Bedroom; Ethel Kennedy buys hers in the Big Apple; Buckleys and Rockefellers collect 'em, and the chic new Nickleby's gift-shop here is packed with 'em.) Suntans. Pedal-pushers. Hot on the Hill: The Cambridge Plan diet. (Sen. Paula Hawkins jolts up the little 330-calorie packet in a blender with ice cubes, now an office fixture.) Carter nostalgia. (The Former First Pair will pop to town for fund-raising hoopla this weekend. Tomorrow, there's the ball at Indian Spring Country Club, and a soiree privee at Gretchen Poston's place.) Joan Mondale's pots. (The pot-happy ex-Veep's mate has been whirling away at her wheel five to eight hours a day. Joan-pot fans know to creep in early to the big pot show at the Toad Hall Pottery next weekend. Last go-round the pot-hounds poured in on the dot of 10 a.m. and snapped up every Joan pot, before 11 a.m.) And class-act burgers. (They dub them "State Department Style" at the Georgetown Hamburger Hamlet, and dish them caviar-style, on toast points with herb butter.) Watch this space.