Quick hotel breakfast then RER C train to Versailles Château Rive Gauche; travel ~1h. Versailles is closed Mondays so Tuesday is a perfect choice — open typically 9:00am–6:30pm.
Choose a cultural stop: Kilmainham Gaol (Irish history, book timed tours) or EPIC (Irish emigration interactive museum). Check opening times (usually 10:00am for EPIC, Kilmainham has set tour times).
Full-day scenic trip: train from Oslo to Myrdal, Flåm Railway, fjord cruise and return — long day (~10–12h) but iconic fjord scenery. Tours depart early, check exact schedule and seasonal variations in April.