Napping for 5-25 min or doing Natural Stress Relief meditation (a cheap variant of Transcendental Meditation) are both things I find restorative if I need a short break, without the risk of finding it hard to stop.

I have the same problem you mention with reading. One thing I find helpful for that is reading on a device where I preset something to block my reader (Chrome on laptop, Kindle app on Android) after X minutes (using Leechblock for Chrome or Appblock for Android).

I've found Beat Saber helpful too sometimes, since it is immersive and gets my endorphins going (improving my mood), but sometimes get overly engrossed in that and end up going longer than planned.

Things that come with a natural time limit: For example, I like playing computer games, but a lot of them has no natural time to stop playing and change activity (by design). Some of them do however. With roguelikes for example, I find that it is engaging and fun to play a round, but after I die there is a natural threshold for starting a new run. Similarly for reading: If I'm reading a novel I'd read continuously for days if I could, but if I read short-form fiction there is a natural point to stop at the end of story. Chapters doesn't do that for me unfortunately.  

I'm notoriously bad at this though, so maybe my strategies are faulty.

Any exercise: generally bound to set length of time or distance (reps/lengths/miles/etc). Ideal lunch break fodder

Walk around block: not that interesting but a good break

Meditation: not as in-your-face restorative as exercise but still good

Cook something 

If co-working: organise breaks to chat (pomodoro or some other schedule). I find virtual co-working, ie video chat with friends or Focusmate/Ultraworking with strangers, is good really effective when working from home (more so with friends than strangers). 

• going for a walk
• taking a long bath or shower
• going to the gym
• taking a nap if I'm tired