Your time.  Spending time with your kids is priceless.

Your kids grow up so fast-- if you blink you'll miss it.  Kids appreciate the attention.  I try to watch movies with them, play board games in winter and tennis/badminton in summer, and I like going on hikes with them.  I especially like getting hot chocolate with my daughter . . .

You can always make more money, you can not make more time.  They do drive me nuts sometimes, but I really do enjoy my time with them.

Make time for your kids . . . you'll be glad you did.