Reading is our go to activity, and we could probably stock a small library. Not only did my parents shower me with books as a child, they saved them all for me, and I have done garage sales and library sales and every holiday, I spend most of my budget on more books. :) My toddler (2) is in fact, right now, sitting in a chair reading books.
Reading is the best...not only does it stimulate them much more than toys, tv, etc, you also get automatic cuddle time the whole time you are reading...which for me is a huge bonus because my toddler isn't particularly affectionate. :) I love to have her in my lap!!
I think that's the best quality time night time activity...I am so biased, though, because I love to read.
Some really awesome books to start with are "The Spooky Old Tree" and "The Picnic" by the Berensteins, "Go, Dog! Go!" by P.D. Eastman, mother goose books, etc...we also recently discovered the Little Blue Truck books and a book called "Goodnight Goodnight Construction Sight" that my toddler is enjoying because she loves trucks. Her favorite book is "Mother Goose in the City" which is a Little Golden Book from my childhood. If you can find it on Amazon, every child I've ever met has fallen in love with this book and memorized each little poem.
When we do play at night, we will play grocery store or sometimes we sit and play video games together. In fact, my hubby just went to rent me a new game...I usually save it for when the kids are in bed, but sometimes we play together. My oldest child used to just love to lay on the couch with me and watch me play Zelda. Not very stimulating, granted, but still some good snuggle time when we'd had a busy day.