Series: Yukkuri in the park (Yuppiyupitaro)

Source: http://www.pixiv.net/member_illust.php?mode=medium&illust_id=59554047
Original title: ゆっくり公園にて

Note: About halfway through the pages, the story splits into 2 paths: The Shelled Marisa and Big Sis Reimu and the Deibu Mom and Wasa Reimu. Summaries for both can be found in the comments.