A day trip should give you enough time to enjoy the destination, not just enough time to arrive and turn around.
Count from door to door
Include the journey to the station, waiting time, connections, and travel at the other end. The advertised train or bus duration is only one part of the outing. Write down a realistic arrival time at the actual place you want to visit.
Know the main reason to go
Choose one experience that justifies the journey. It could be a particular town, landscape, or collection. If you cannot name what you want from the outing, another day in your base city might be the better choice.
Check the return before leaving
Look at current operator information and leave a margin before the last practical service. Do not build the plan around a single tight connection. Save the relevant details where you can access them even if reception is poor.
Keep the destination plan short
Choose a compact route and a meal break. Avoid adding several nearby towns simply because they look close on a map. A good day trip should feel like time spent somewhere, not a series of departures.