Hit yourself on the fat part of your thigh with one of these, a rubber garden mallet. Hurts like hell, but since it is made of hard rubber and the head is very wide it only causes (at most) minor bruising. Rubber and a wide face were my reason for using this specifically and not a regular hammer, and why I would suggest it now. And
not suggest a regular hammer. In any event point of contact is in an easy to cover up part of your body should it bruise. The only downside I recall is at one point having such sore muscles I collapsed in the shower with my legs just giving way after a particularly harsh session. But only once.
I did this for about six months many years ago. I would stand with the mallet at arms length and slam it into my thigh, about ten times to each thigh. Made a sound similar to tenderizing meat. Hurt every bit as much when I stopped doing it as when I started, so no escalation of the practice was ever needed. Not sure why I started but I stopped because after a while my dog would follow me into the basement and kind of freak out as I would hit myself. So I stopped. Once she passed away I never got back into it.
I
might have done some nerve damage to my left thigh, but the evidence here is kind of sketchy. The skin there is always peeling and dry and there isn't much feeling. But I think this might be the result of a medical condition I have and not this since it didn't manifest until years after I stopped the practice of beating myself with the mallet.
Obviously the main reasons for my approach here were to (a) avoid discovery, (b) avoid permanent disfigurement while (c) causing myself a great deal of physical pain. All three goals were met, other than with (a) the aforementioned dog judging me.