In summer it is truly unmissable, even if you are passing through. Take a small towel with you, don't miss the chance. The sea is truly crystalline, sensationally clear seabed. Don't forget your sunscreen though, no shade!
Don't worry, you can always reach Manarola on foot, along the ancient mule track that the inhabitants used before, the Via Beccara, now marked as path 531. It is "only" longer and more tiring .. an hour anyway and what views.
Perhaps this point should be number one. In one of the small bars in the village, you should not miss a nice slice of pizza, focaccia and farinata while you are walking around Riomaggiore as a DOC tourist.
Speaking of DOC, you can't really not try the Cinque Terre wine, have you seen in the terraces around you? Heroic viticulture they call it, how to blame him. With a nice dish in one of the restaurants in the village, the cuisine here is very tied to the hills.