In Monaco, you can almost feel it's steep history seeping into your veins. Being the second smallest country in the world after the Vatican city, it's so beautiful and cozy, and the locals are very much friendlier than their northern neighbors who share the the same language as them. Being so tiny, most of the tourist places are situated near each other, so it's highly recommendable to explore on foot.
The Aquarium & Oceanography Museum which houses one of the oldest operating aquariums in the world is one of Monaco's principal attraction. Another not-to-be-missed attraction would be the fortress-turned-palace, Prince's Palace of Monaco, and witness the ceremony of the Changing of the Guards at 11:55 a.m. sharp. It's also a delight to ramble in the crooked and narrow streets lined with mostly souvenir shops just in front of the royal palace. And what is a trip to Monaco without visiting Saint Nicholas Cathedral where the Grimaldi family vault, in which Princess Grace was buried, is. And I agree with TheLifeExperiment; you should definitely check out the awesome yachts.