If you're staying in Balloch as you mentioned upthread as a possibility, you could hike this trail to Helensburgh and have food and/or drinks there. There are plenty of places to go in both Helensburgh and Balloch. Then 10 mins on train from Helensburgh to Dalreoch and 10 mins to head back to Balloch (they are on the same line but the line splits at Dalreoch).