We spent a couple of days in Gros Morne during our 2 week visit to Newfoundland. Although we're not as active as the ardent hikers, kayakers and such, there was still lots to see and do.
The West Brook boat tour is definitely worthwhile but the 45 min hike to and from the boat (each way) might deter some. The Bonne Bay boat tour is a perfect alternative and for couple, take advantage of the Sunset cruise.
Visit the lighthouse at the north of Rocky Harbour and if you can handle a short hike, there is an observation point just before the lighthouse with a relatively easy 700 meter hike up to the top.
There's a short hiking trail that leads to top of Southeast Brook waterfall. Nice hike and view for those less inclined to strenuous trails. There are several observation point along the highway that give nice views of the majestic landscape.