Keep Them - If you're still planning to have another baby, then the most reasonable thing to do is to keep those old baby stuff. This will help you save money on buying new clothes, toys and other baby things once the new baby arrives.
Donate - There are a number of charitable institution who takes care of abandoned babies and children. Your child's old baby stuffs will be put to good use in these institutions. There are websites who connects people who wants to give out baby things to those people who need these baby things. Just google search "free baby stuff" and you'll find a number of website who does this.