@Miyono-chan happi is summer overcoat, traditionally worn by men. (but it has no length what so ever) it's the one that you see during the festival episode. hakama is a pants layer, usually made out of a thick fabric and- ummm... is usually just underwear underneath. it's suppose to be loose fitting to not restrict posture, usually for archery or kendo.
kimono can just refer to a patterned overcoat, which is how it's being used in this chapter. but it looks more like the modern kimono where it's just worn as an overcoat; without the other flair such as a sash, hair pins, etc. the full kimono is usually used for traditional reasons as well as costumes.
I'm not sure why the author specifically draws it like that (but I'm unaware if the modern movement is just a replication of past designs. and the author is just representing it during the appropriate time period).
kimono is just specifically an overcoat with long sleeves that aren't as long as furisodes (sleeves the drape far enough to touch the ankles), or tube sleeves. kimono can be worn by either gender. sometimes they seem shorter because the sash and the skirt is tied up that high. very much like the high waist of a certain European period