Pain without marks is definitely doable, but to really push it you have to understand the bottom.
You can use irritants like icy hot, bengay, or the like on the naughty bits - be careful because a little goes a long way, and most of them last 20-30 minutes and once on you can't really get them off (pun intended).
More info here:
https://www.theblackpomegranate.com/...chemical-play/
Electro as previously mentioned. You can pick up a TENS unit for cheap, or if you really want to go pro grab yourself an
Erostek or for the true electric aficionado grab a
Centipede - I really want to get my hands on one of those, the nerdy do-it-yourselfer in me loves it. Hopefully someone will figure out how to link it to VR and really kick it over the top.
If you are wanting pain without marks, impact play gets really tricky, but you can always soak a cotton towel and place it over the buttocks and then use a cane, that can prevent a lot of marking. But any time you do impact play and are wanting to avoid marks, it's probably best not to.
There's also pressure points, some of them can be very effective.
This should get you started, or just grab any martial arts book on pressure points and that should give you some great ideas.
Anyways, hope this helps!