Na-i-sho is a 13 episode OAV series detailing the summer between the ~motto~ and the Dokkaan season. It is a bit more mature than the TV seasons, and looks to develop each of the five witches romantically. Hana is back to a baby, and the mahou shop is a bakery.
Ojamajo Doremi Naisho, placed somewhere in the summer vacation between the school terms covered in season 3 and season 4, is practically a loose chain of short stories around the characters of Ojamajo Doremi. While completely knowing the parent story is not really nescessary, it is probably useful to have watched some episodes of the parent story. (I only watched the first two seasons and about half a dozen of the other two and that was plenty enough. Even half of that would be enough.)
Ojamajo Doremi deals with five girl "witch novice" protagonists. While the parent story mainly focuses on these girls, here they actually
play more of a supporting role. The spotlight in each episode is reserved on some side characters. Some of them being comepletely new, some other are also featured as (more or less minor) side characters from the parent story. In each episode the character's problem is featured, with a part of the main charaters stumbling over the the character and more or lesss secretly goes on helping the character or is observing his or her character development.
While magic is still an element of the series, it has not really that much weight in the series. These shown problems are usually not dealt or tried to be solved with it, but by the characters themselves.
In addition some quite serious and, considering the audience of this franchise, surprisingly mature problems are dealt with in this series.
The series is surpsingly realistic in showing and dealing with these problems and I really like how magic is not a direct part of solving these troubles. The series is at its core very mature, but still presents the topics in a manner, very suitable for children. There is quite some educational value inside of it.
Despite its age it doesn't really look dated. Due to its parent series, it's still quite trimmend on "cute" and very colourful, although, it appears to me, not quite as much as in the parent story.