Make the east-bank ending optional
Sumida City lists Sumida Park in Mukojima one, two and five-chome as continuously open, with no regular closed day. Individual facilities and temporary conditions still need separate attention.
Park Center describes an approach across River Walk and names paved and lawn Soyokaze plaza areas and a Japanese garden.
Allow time for a park pause.
Choose a stopping place that is permitted and works for your group on the day. A lawn on a map does not by itself settle where you may sit or what kind of activity is allowed. Keep the rest break simple: decide whether it is helping everyone enjoy the outing, and finish here if another destination would be more effort than pleasure.
Local tourism lists Ushijima Shrine at 1-4-5 Mukojima, and its walking-course page includes both the shrine and Sumida Park.
Treat the shrine as a separate choice, not a required finish after the park. Check visiting conditions before making it the reason to extend the walk. If your interest has shifted toward dinner, a rest or the return journey, let that be the ending instead.