Europe is not an ideal destination to work or volunteer on your gap year. You'll find that opportunities are a great deal scarcer than you would expect if you travel to Asia or Australasia, though it is possible if you look hard enough.
Almost every paid job for travellers in Europe comes in the form of teaching. These jobs will pay reasonably well, and sometimes come with accommodation included, and it's experience that looks incredible on your CV. You'll definitely need a Teaching English as a Foreign Language (TEFL) qualification, and often a higher qualification such as a CELTA or advanced TEFL. You will also have to have a degree.
The approach many take is to teach elsewhere in the world, such as Asia, to get some experience, and then use this to get a position in Europe.
Volunteering options are also limited in Europe, centering on professional internships and work experience. This means if you're looking to bolster your CV, Europe can be a good option. A fairly wide variety of volunteering positions are available, including journalism, finance, teaching, physiotherapy, and others.
It's fair to say that volunteering in Europe is a little more 'selfish,' compared to eco volunteering in other parts of the world, but there are some opportunities to work with children and poor communities, especially in the lesser-known countries of Eastern Europe.