Hello there.
My little girl is also 21 months old and we do a lot of what Annieko mentioned.
She loves to be outside. Enjoys going for walks, climbing a jungle gym, bouncing on a trampoline, riding her push bike, pushing her pram or trolley. She really enjoys bouncing around on an inflatable cow that we have.
She's also very into jigsaw puzzles, loves Counting Bears, pouring, play dough etc.
A website that I really like is
www.1plus1plus1eqauls1.net I subscribe to her blog and website and receive emails of her posts. Her children are older but you can search for posts when they were younger and there are a lot of pictures which give you great ideas.
She enjoys Little Reader & Little Musician software, Monkisee DVDs and flash cards and Starfall.com
Enjoy your little ones!