Keys

Let's not forget the all time classic toy...

Music (Shake it all about: pram, high chair, car seat, picnic rug....)
Manipulation (separately discover them, shake them altogether, practise holding like you are opening a lock...)
Sensory (wonderful cold, metallic, odd shaped pieces that jingle and jangle about...)
Counting

...Just don't lose them! (...alternatively, it might be worth keeping a few random unwanted/unused keys designated for play)