Stirling and Edinburgh are a little further, but both have excellent castles to visit. Culzean (pronounced Cullane) has a really good adventure play park and has lovely walks, on a sunny day it would be a really good choice. .
We live not too far from Greenock, so feel free to ask anything else. We're planning to pop down when Magic is in port, to show our wee boy the boat he'll be going to Spain on.