An MVNO of AT&T in the US is called Cricket, and they offer a month to month plan with 2.5 Gb of data for $40 a month. If you have automatic payment, it drops to $35 a month. Their data is LTE, but limited to 8 Mbps, but that's pretty decent, and will allow streaming videos, and good download speeds.
I don't know if they will allow you to buy their sim from Europe, but if you know someone where you'll be staying, perhaps they can help you out with the purchase.