My daughter, and second child, is now 5 years old, but still occasionally regresses. There is definitely a need for closeness and attention involved, and I believe in giving it fully. Even to the extent that she is truly treated like a precious baby (just during the night ritual) with a devoted Mom to hold her and talk baby talk. Soon, it gets so syrupy, she asks me to "Stop IT"
But I really only respond to positive "requests".
Whining is responded to with a lack of emotional response or eye contact. I simply say, "I do not understand the language of WHINE. Please talk in your sweet voice." As soon as she uses an appropriate voice, I look at her in the eyes and pay full attention to her request. I try to "see" past her immediate request to the underlying purpose of the request. I read once that most all of children's behavior is based on two needs. One to GIVE love / attention, and the other, to GET it. Sometimes I have to guess. Other times, I'll ask, "Did you want to share something with me?" or "There are five minutes before the laundry is done, did you want to tell me something interesting, do you want to draw a picture I can hang on the fridge, or do we need a quick Tickle Time, before we take care of the clothes."
At 21 months, sometimes it is difficult for them to participate in many responsibilities. BUT understanding child development, and making it into a game, can help. That age is focusing on understanding categories. Call out the category, like "towel", and then count out loud how long it takes them to find one in the basket and put it in your lap (this adds the fun race factor).
After about 3 years old, both my kids loved matching and folding the socks, into sock bombs they can throw at me. Children this age are processing humor, and love to be the one to get others to giggle. Oh, the amazing things we agree to do as mothers. But... well... whatever works!
As far as self-entertaining, just keep trying new and different options. I highly recommend hands on manipulative materials, such as playdough, blocks, legos, or yarn, with high quality music (like Mozart, or even Kindermusik) playing in the background. This engages their mind, as well as their hands. Do it with her the first few times for the whole time, providing support for her ideas, as well as encouraging new ideas. Then gradually "go to the bathroom" and be right back, and so forth and so on. As she grows older, you might let her choose the recordings and add a selection of audio books (appropriate for her age). My daughter LOVES them !