The best advice would be to check out bars, restaurants, and cafes. A lot of these places need bussers or other staff. Also, some hostels exchange free stays for working there. I think Flying Pig in Amsterdam and Pink Palace in Corfu are two of these. Also, depending on when you go, resorts hire people who can speak English and also tour guide work can be found. Teaching English would probably be for someone looking to stay longer and is tougher to get work with a reputable place without a TEFL degree or the like.
Just remember, when you are working under the table that if you get stiffed by some employers, you are out of luck. Since you are working illegally. Best of luck!