I can confirm Czech Republic and Spain as both being pretty straightforward and simple, only because I've lived both places and known people who have gone through the processes. I'd be surprised if it wasn't doable and relatively easy in the majority of EU countries. You'll just have to google the country you're interested in along with "business license" or something similar.
Getting a work visa or immigrating into the United States is so difficult that I think it's hard for Americans to imagine, but it's actually pretty easy to work legally in the EU if you're able to get a decent job (and by that I mean that McDonald's isn't going to be hiring foreign employees and sponsoring visas) or obtain a business license (usually some fee for the application, proof you can perform the job you say you're going to do, and the onus of administrative duties related to taxes, healthcare, etc).
Plus, you can get creative. I was issued a 5 year "EU family member visa" just for having a Czech girlfriend. This gave me all the rights of movement and work that EU citizens enjoy.