For my 4 yr. old daughter we threw a Bubble Party. I sent out invitations made on paper that where you blow colored bubbles onto paper with a straw to make the bubble impressions.  For the party I made a cake and used the clear plastic domes that come in coin prize machines (like in the grocery store) and put party stickers on toothpicks inside them so it looked like the birthday images were inside of bubbles.    I made a bubble pinata in bright pink.  For the favors my dad helped me make about 50 big bubble wands out of dowel rods and cotton cording that could be used to make huge bubbles.  I had 4-5 big buckets of bubble solution made with glycerin, dawn detergent and water for the kids to play with.   We had bubble art with colored bubble solution. The kids had a great time.  We had a bubble machine I borrowed blowing the entire time with just tickled the babies in trying to catch them.   A bubble party was perfect for my baby girl.    Have a great party!