Cell phone plans for US citizen living abroad in Europe for 3 months?

I will be living in Barcelona, Spain for 3 months in the summer and I want to bring my cell phone with me from the US and use it while I'm there. I have an iPhone 6 that's out of contract with Verizon which I will unlock before I move.

I will be traveling within Europe occasionally, every other weekend, and I don't know what will be my best strategy for having a cell phone plan with adequate data (4+ GB) while I live in Spain as well as when I travel to other European countries.

Most of Europe's plans now include full and free roaming throughout the EU.
and the EU parliament put tight limitations on the cost of calls/data when roaming anyway

Come summer of 2017, roaming charges are supposed to be abolished completely.

Spanish networks
http://prepaid-data-sim-card.wikia.com/wiki/Spain

Just get a SIM when you arrive.
Best place - any Vodafone or other shop in Spain
yes, you may get one at the airport if they are promoting them. But best at the shop

Vodafone.es
http://www.vodafone.es/particulares/es/
SIM ONLY
https://www.vodafone.es/tienda/particulares/es/movil/solo-sim/

So any 30 day plan will be fine.
even a PAYG (Pay as you go or US 'prepaid' plan) SIM will do.

teh ONE THING you will need to do
is make sure you have unlocked your phone before you leave
That usually means putting a different network SIM in… To force it to unlock (and you may need to sync it again with iTunes) - so get a different SIM (borrow a friends?) to check / do that before you leave.

If you need to pay to unlock - a small fee to Apple, so be it