Answer: sightseeing
Step-by-step explanation:
Opportunity costs represent to the potential benefits that an economic entity misses out on when another alternative is chosen over another.
In this case, due to the increase in the price of the plane ticket, Mikael decides to give up visiting the popular Ouro Preto during his stay in Brazil even though it was earlier planned. Therefore, it can be infered that sightseeing is the opportunity cost as that is what he forgoes in order to choose a different alternative.