The Tagus River is definitely a must in Lisbon. And there are so many ways to enjoy it.
You can ride a bicycle or walk in several parts—in Belém or at Parque das Nações, or under the April 25th Bridge. Join the fishermen and get some fish; sit at Ribeira das Naus, enjoying the view over the bridge and to the other side of the river; maybe rent a sailboat and venture out into the river. Or, if you prefer, go canoeing.
Have a drink or a meal next to the river, then take the ferry to the other side and see the city from outside.