I just sit and feel my contact with the surface I'm sitting on. No concepts or ideas, just the reality of my weight on the bed or chair I'm resting on. I find it pretty grounding.
I think these things can be more than temporary, but they are not quick fixes either, more like developing habits. You have to think that if you are dissociated then that is the dominant state. Splashing water on your face or just sitting every once in a while isn't going to change that. But if you do these things every day they could have an accumulative effect in helping you to reconnect with your body and presence in the here-and-now.
Say if you look at your hand. Do you automatically see it as an alien object, or do you automatically see it as part of yourself that belongs to you? Both of these attitudes are habits that can be cultivated. That's the idea, anyway. Worth a shot.