At 21 months your toddler is probably showing more signs of symbolic play (pretend play or imaginative play.) During symbolic play your toddler may play doctor on her stuffed animals or “drive” a shoe-box around like it is a car. This shows a higher level of thinking as it uses creativity and the ability to think symbolically. Encourage this whenever you can. You can even model symbolic play if your child is not engaging in it yet. Pretend to drink from a play teacup then say “Ahhh.” Then offer the cup to your toddler.
She is probably taking interest in children playing beside her and she might mimic their play and even pick up the same toy and start to engage. This is called associative play. At this age, many children are starting to move from parallel play to associative play. She will still need some help with social skills, but play time is a great time to practice that.
References:
Hirsh-Pasek, K., & Golinkoff, R. M. (2016). Becoming brilliant: What science tells us about raising successful children. Washington, DC: American Psychological Association.
Hirsh-Pasek, K., Golinkoff, R., Berk, L., & Singer, D. (2009). A mandate for playful learning in preschool: Presenting the evidence. New York, NY: Oxford University Press.
Fein, G., & Rivkin, M. (Eds.). (1986). The young child at play: Reviews of research (Vol. 4). Washington, DC: National Association for the Education of Young Children.