Arakawa Under the Bridge, Mawaru Penguindrum Recommendations
Both shows are wonderfully weird and quirky. They also have similar art in that they're very colourful, pretty and nice to view. Although Mawaru Penguindrum is much more serious in parts, it has a similar air and feel to Arakawa Under the Bridge. :)
While completely different, Mawaru Penguindrum occasionally has a similar tone to the humor. There is also a very strong Shaft quality to Penguindrum. Both are highly enjoyable, and I recommend them both.
Very similar art styles, especially in character models, and unique in both story telling and characters.
Also has the same opening artist (Etsuko Yakushimaru) which set a similar atmosphere for both.
Penguindrum is more pessimistic in its story telling while Arakawa is more silly and optimistic though.