I think the massage certificate is a cute idea, but I think if the host wants to do that, then it should be the gift from the host. Its not really appropriate to insist that the guests pitch in for a group present without it being cleared with everyone before hand. They are already buying a shower gift. Its a little rude to ask them to also help pay for a massage that wasn't even their idea.
If you insist though, I think the best way to word it would be "Donations will be accepted in honor of the mother-to-be for one pregnancy massage session at such-and-such spa." Then at the party, you can decorate a jar to match the decor of the party. Any guests that can donate a few bucks can drop it in the jar, but there's no real obligations.
The only surprise baby shower invite I've ever gotten had a front that was decorated in tons of cute baby stuff and it read, "Its a......" and on the inside it read, ".... SURPRISE!" Then there was a short greeting that said, "You are cordially invited to the surprise baby shower for...... etc, etc, etc".