Cute stuff like this makes me jealous because my daughter was born in December -- the summer kids get to have all the fun! Maybe I'll start celebrating her half birthday.
You could also use these little balloon boats as favors or have the kids decorate them as a party activity ... the options are endless.
Go to Oh Happy Day for the details on the invites (you've got to see the cute little box you can send them in).
Are you having a summer birthday party for your toddler? What's the theme?