It depends what you mean by “European flights,” “costly,” and “hassle.”

Do you mean flights from N.America to Europe? If so, then standby can be done. It’s called “unrestricted fares” and airlines will be happy to sell you these tickets, typically $2000+ for a round trip. If a seat is available, you fly. But you pay dearly for the privilege of this flexibility.

If you mean intra-Europe flights, then generally, no. You can always buy a last-minute seat, but it will be at a last-minute price. Even uber-cheap Ryanair can be 200 Euros or more if you wait to book within a day or 2 of departure.

There is an “air pass” program called EuropeByAir.com. $99 one-way + tax to fly standby on select routes. Problem is, if you know just a couple weeks advance your route, you can sometimes still snag the Ryanair or Easyjet fares for like less than 50 Euros all-in.

US Passport holders must meet entry requirements. See the “nifty visa checking tool” thread pinned @ top of this forum. Note “Schengen” acts as a single border control for most of western Europe. Once you’re in Schengen, then there aren’t further border controls for travels within those countries. Of your list, only London is not in Schengen.

For cheap flights within Europe, see Skyscanner.com.

Thank you! yes i meant flights within europe. this info really helps to clarify things.