Sometimes I give the girls a big box of magnets to play with, but usually they just line them up on the cookie sheet or put them in containers (which is fun too), and ignore me when I try to make it into an organized activity. They do, however, love when I give them smaller-scale challenges on small magnet boards. I'll make up different tasks for them and keep it down to a reasonable size. For example, here's an activity I put together for matching upper case letters to lower case ones (the magnets are Mellisa and Doug):
Here's another magnet activity board I put together, where the girls need to match each object to its beginning sound:
We're still working on beginning sounds, but they get the concept. The other day, when L got out of the bath she said "I'm cold.. cccccc. Cold starts with 'c'". These kids soak up new concepts like little sponges.
Anyway, these magnet activity boards are so easy to set up that it hardly takes any time at all to switch the activities around. Since the boards are dry-erase in addition to being magnetic, I want to start having the girls draw lines from the objects on one side to those on the other side, but we're not ready for that quite yet.