IMHO, if you're doing Peace Corps, TFA, AmeriCorps, etc. primarily to increase your chances of getting into law school, then I suggest you stay the extra year in school. The GPA boost is probably more important in most cases.
However, if, as you say, you're looking for something "meaningful," then any of those service programs would be a fantastic way of getting more experience. If you find you don't like school but that you are attracted to the kind of service made available through these programs, then go for it. It will intangibly help you with law school admissions, but, more importantly, I believe any of those programs will make you a better human being.