A shelf or a windowsill with a few objects on it — a candle, a bowl of water, a stone, whatever the season is doing — reset at each new moon.

It is the least doctrinal thing in modern lunar practice and probably the most common. What it actually does is put a recurring appointment somewhere you will see it, which is the mechanism behind most useful rituals: not the objects, the fact that you stop and look at them twice a month.