Variety! Lots of different kinds! Rotate the choices: turn the box inside out once in a while and hide some away so they look new again. I've met dogs who have only three or four toys--really!--and they get into trouble a LOT. Go figure. Would your human toddler get persnickety if she had only four toys? Boring! One of my favorite things to watch is a dog looking through the box, pawing through toys to find the one she wants. Adorable!
These puzzle games aren't inexpensive--though watch www.jefferspet.com for great deals--so I lend them out all the time. Dogs love them--until they're too easy to solve. Hide them away and then bring them out again after a couple months. Ooh, new toy! You're the best dog parent ever!
Then have more fun!