Others are far more expert than I, but I believe you can sign up for an AT&T sort-of-unlimited $60/month and then you can cancel with no penalty within 30 days. That's still pricey, particularly with the activation fee.
If you can find a used Verizon USB modem, I do know that there are lots of folks selling Verizon refills on Craigslist for 30-50% off of list price. You could then get a prepaid week's (or month's) data that way.
Depending on where you are and the coverage, Cricket --
www.mycricket.com -- might be another option. $40/month with no contract and free modem.
As to whether ANY of these will work in Europe, I have no clue.