A warm spring day full of sunshine had youngsters doing what youngsters have always done on such auspicious occasions: taking to the trees, in this case at Hidden Falls Regional Park in St. Paul, conveniently located next to the daydream-inspiring Mississippi River. Above, Andrew Green, 8, of St. Paul worked his way along a big branch. At right, James Cambronne, right, 5, and Archie Johnson, 4, both of Minneapolis, watched the river nestled in a perch. Not to spoil the fun, but the forecast includes, um, flurries. B8