Picnics in the park make the best filler. beach episodes, hot springs episodes, picnic episodes, wacky races episodes. all of these are fine and enjoyable. the filler i can't stand is injecting a filler villain or threat into a series and making the characters develop new moves or strategies to beat it that will never be used again. that, or stretching out an arc that's alredy on a timetable with filer episodes that flub the passage of time from the manga.