Have stayed on Saba and Nevis, done a day trip to Saba as well (ferry).
If day tripping to Saba recommend the plane for the experience. You'll have time for a guided hike to the top of Mt Scenery (need to be reasonably fit for that) or look round one of the towns (villages).
Can't see much point in a day trip to Nevis but we enjoyed a week there, lots of good restaurants, the capital, Charlestown is very attractive. And we visited the Hamilton museum, admission was...a Hamilton (for two of us, 5USD each).
Our trips were all pre-plague so no idea on current restrictions.
Dave.