Well, if you're having a rest and lying down, it could be very helpful to focus on the breathing as that's keeping the practice going. Sometimes if you are trying to do reflections while you're having a rest - it could stimulate the mind and you're trying to have a rest. So if you happen to be lying down, you could do the deep relaxation posture and watch the breathing, that would keep your mindfulness going.
The reflections going? If you feel motivated to do reflections, perhaps keeping it to joyful subjects might be more helpful than doing reflections on death or Dukkha or these types of things, because these may stimulate the mind too much. Keeping it to reflections that are more neutral could be very helpful. If we're walking somewhere often it's very restful to be able to walk and be resting in your feet. And that is the mindfulness resting in your feet. You feel much more rested by doing that than if you allow the hindrances to take over while you're having a rest. I wouldn't suggest doing standing meditation while you're having a rest as that may be too exhausting. So keeping it to postures that are restful can be very helpful.
Another way that we can actually be practicing without feeling that it is a practice is, say we're at the lookout and there are clouds, simply watching the change in the clouds. Sometimes we're just looking and we are just looking at the scene without actually going a bit deeper, but sometimes it can be very calming to watch the changes in the clouds. Seeing that things are not static, so the internal/external sight type of reflection can be very helpful when we're having a rest. To allow ourselves to see things but with a purpose behind it. To watch, to observe things closely. It can be calming but it also can be giving us a great basis for reflections when we're doing it in a much more intense way.
For example - a story I heard was about a monk going on alms round and he started focusing on the harvesting of the rice, watching, observing, and then he focused on the rice seed and from that, allowed himself to see what was going on. He began a contemplation about the germ of the rice seed and how this rice seed then grew more rice seeds, grew more rice seeds and then he started to likened this to the cycle of birth and death and the sense of becoming to the germ within the seed. So sometimes by allowing ourselves to see things while we're resting, we can use the power of our observation as way to begin a formal contemplation on Dhamma.