For me, the main attraction of Rosario during the summer is the Parana river and its associated water activities.
To name a few, you can do:

Kayak

Go on a boat ride to explore the Parana Viejo

Go to the beach

Wind surf

Go sailing

Go speed boating

Here is a great summary of some of the things to do with names and contact details of the companies offering these activities on the website of Rosario Tourism Office.

River, Beach and Island Activities

Enjoy your stay in Rosario this summer!