Oh, come on. Spain begs for massive wars! But if you want to go the diplomatic way, try this. Build up a huge army and navy all over your empire, all the while befriending and diploannexing other nations. When finally your BB gets so high that the world DoWs you, that huge army kicks in. Rampage across the world!
Also, during this time, resist the traditional spanish urge to colonize. Try to diploannex the major colonial nations first (England, France, Portugal). You could probably even just militarily annex Portugal right off the bat. The minor colonizers probably won't have any colonial cities, meaning you can take their mainland provinces with no fuss overseas.
And by the way, diploannexation, although it does increase BB, it does not do it to the same extent. If you search around, I'm sure you can find a list of BB costs.