Doo-dah is a period-21 spaceship consisting of a weekender pulling a period-21 tagalong that was found by John Winston Garth on December 6, 2020. In its simplest phase, the tagalong part consists entirely of two R-pentomino descendants. The spaceship was found using a combination of ikpx2 and apgsearch, with partial results from the former being inputted into the latter via ikpx2's "standard input" feature, with the goal of searching higher-period spaceships and puffers. Garth named the tagalong "doo-dah" after a nickname for his grandfather.

A beehive puffer based on this spaceship was found the same day, seemingly independently by iNoMed and GUYTU6J. GUYTU6J also found a block puffer formed by adding a second weekender to the side of the p21 ship. Both of these puffers can be either one- or two-sided. Various glider rakes have also been constructed. On December 7, GUYTU6J and praosylen found several period-42 doo-dah-based spaceships.