Freelance work overseas for short time

Hi, I'm having clients wanting me to fly to Europe to provide a service for a very limited time. I'd like to make good use of my time there and make a little tour of sorts and have a bit of a vacation at the same time. I'm talking only a few weeks and two months max. Potentially I'd be going into a handful of different countries. It seems that work visas and such involve me wanting to live there and stay working there beyond a 90 day period. Are visas and permits needed for such short term work if I'm still living in the US and not trying to get a residence overseas?