Mousies (as distinct from their living mice counterparts…) have always been great favourites here well, once we got the concept of “toys” established, anyway… to the point of some getting Loved To Death. Jingle Mousey [pictured] (who was, er, pawed down from a friend’s cat about 9 years ago) has survived though…
I think having 2 can make difference if they’re 2 who play together as they may have more interest in balls for without you than a solo kitten would. Though obviously there are probably some lone felines who are busily building ALL the ball skills when unattended & pairs who will look at you as if you have suggested they try swimming on being proffered a ball to play with.
On subject balls, wee foil-type ones like these are greatly favoured here because they are so light - perfect for batting about & thus losing under kitchen appliances but better still (according to a certain blond cat) perfect for playing catch with 
My cats lack the catnip gene, but adore valerian, so they have some valerian toys, which occasionally get refreshed with valerian spray.
Sisal toys are more popular with my (cardboard-chomping) black cat than his brother; & he has a rope of owls (as one does) he quite likes the occasional chew on. Both quite like the chew toys you can get for cats (despite the catnip thing, yes).
Have fun 