That's pretty much how I do tourism: go to a high tourism city/region (where the local culture has already learnt how to deal with it), then ruthlessly demote every popular "must see" to a "might see" and see what else is there. Some "must sees" will still be visited, but in a purely opportunistic manner. Turns out the colosseum isn't a complete waste of time if you randomly passed by at a time when there wasn't any queue at all, but if that does not happen there's no shortage of other things to see.