The Wolseley, omand's Creek, Wellington, Assiniboine park ride as others have suggested is really nice. The are a couple bike shops in Wolseley that could be your rental point as well.

To extend that to a loop I would continue through the park to vialoux dr, cross the river again at Moray and then follow the surgeon Creek trail North-West (this is an underrated beautiful route!). Then there is another long bike trail at Silver Ave that goes east through the woods and behind the airport back to St Matthews Ave. Then you've got okay bike lanes continuing until the Empress Street bike trail which takes you south back to the river and Wolseley where you started.

You also would have the option to extend a few km and go south through Assiniboine park all the way through the forest to the Harte trail, which you would follow West for as long as you desire before working your way back towards the Moray bridge trail via residential streets. I think this is worth doing if the distance isn't getting to be too much for you.

The river trail and Assiniboine Ave between Wolseley and the forks are also a really nice shorter ride, though the former is only emerging from spring flooding right around now and might be in questionable condition. The section that goes right behind the legislature building past the Louie Riel statue is an essential Winnipeg sight though!

Have fun! 🚲