which port -Greenock? Assuming it is Greenock, the trip to LL and Inverary is a breeze - across the Erskine Bridge and an hour up the road to Inverary. Legendary tearoom in Luss on LL and great fish and chips in Inverary.
I have shown many overseas visitors around and they pick countryside every time.
Unless you paticularyly want to see the Dali in Kelvingrove - take the High Road to Inverary. Our weather is normally kind in May