I am not sure you can get across with a baptismal certificate. The guys at the border need to see prove of citizenship. That is why they need a birth certificate or passport. If you aren't going until July you still have plenty of time to order either one of those. Personally I would suggest the birth certificate, because you will need that to get the passport.

But have a fun trip when you do come!

BTW, PEI is not the only province that issued laminated birth certificates. I have mine from Saskatchewan and it came laminated.