On a budget, weigh it — Monaco / Monte-Carlo isn't cheap for what it is.
Day-trip cheap by train (~€5); spending high
A surreal vertical city of glittering wealth: the Belle Époque casino, the changing of the guard at the palace, the harbour stacked with megayachts, and the excellent oceanographic museum. The honest catch — it can feel like a sterile open-air showroom for money, it's expensive even to park, and the 'glamour' is mostly to look at, not partake in.