I think most medical specialties can give a good lifestyle, especially the ones that are frequently paid per hour (ER, anesthesiology, radio, etc) and ones that have a high % who go into private practice (ENT, derm, allergy, pain, podiatry, etc). I've met quite a few ER or anesthesia docs, even many who are fairly young, that work only a few days per week and are fine having less income in exchange for more free time.
Specialties who are more typically salary FTEs of a hospital (g-surg, IM, ID, path, etc) might have a tougher time finding part time work since most hospitals want full timers since they probably give physicians the same nice benefits package whether you work 25hrs or 65hrs per week. I'm sure you could still carve out a fairly lax schedule in almost any specialty if you wanted that and were willing to take a pay cut. A lot of docs just realize it's hard to turn down patients who need your help (and their insurance will pay big bucks).
It's all what you decide to do, though. Depending on what you want and what kind of training you acquire, podiatry can be M-Th 8a-3p office trimming corns and calluses, or it could be 80hrs per week running a residency, doing research, and taking trauma call.