Gintama Movie 1: Shinyaku Benizakura-hen

A lot of series have recap specials/episodes that recap a season, half of a season or maybe multiple in one? However, these decide to take that approach by not only simply remaking one arc but reanimating and pretty much just recreating everything in the process. These may not add a crazy amount more but they do a solid job retelling the story of an arc in movie format rather than episode format like how the series did them. 

Both of them are adaptations of specific arcs of popular gag mangas. Additionally, there are the overall similarities between the two shows - very comedic, with focuses being on the quirks of characters and the special quirks of the MCs, which are both lazy and sweet-toothed but powerful and sometimes... kinda attractive! And, they're both not that long to watch, but you should watch them in the actual order of the series (for Gintama and SK).