I do computer training for £25 an hour in the evenings. It's been quiet lately but it was ridiculous when I first started. I worked solid every evening for about 2 months £125 a week isn't bad at all! Most of the time it was old ladies going on about their gardens etc etc. That said I do fortunately work for quite a reputable company and was allowed to advise our customers it was something I did. I haven't advertised or anything though.
It depends entirely on what you're good at. I'd have thought computer training, general DIY, decorating, washing cars - those would pretty much be the best ones to do.