There are some nice scenic hikes, a few lakes and some limited wildlife viewing. We stayed in park for 4 days, after 4 days in Yellowstone.

Some of the so called "attractions" are nothing more than tourist traps.

In my opinion, GTNP and YSNP worth a ONE visit, and that is mainly for geological points of Yellowstone.

Considering the travel expense and flight time from California, we take any island of Hawaii any day, for their truely unique nature, and visitor-friendly people.

We spent four days at Grand Teton (Jackson Lake Lodge, review posted) and absolutely fell in love with it. Going to Yellowstone afterwards was more "interesting" but less satisfying in many ways. Grand Teton is just simply gorgeous and while there aren't as many "attractions" as Yellowstone, there is so much to see, do and so much to enjoy that I wish we had had a couple more days.
Other than the requisite float trip on the Snake River and Horseback Riding, we filled our days with the boat across Jenny Lake and the hike to Inspiration Point which was fantastic (our 9 year old made it only half way but our 11 year old led the way for me up to the Point). We also went on various other smaller hikes.
One of our favorite days there was a trip we took out of the park to the area of the Gros Ventre landslide. This has to be seen to be believed. Find Antelope Flats Road and take it out through buffalo filled fields and fantastic scenery and follow the signs to the short trail at Gros Ventre. It was so "off the beaten path" and so beautiful that we all loved it. We also drove through the Teton pass from Jackson Hole into Idaho and then back which was just gorgeous.
This place is almost paradise. Yellowstone is more unique and there's more "to do" but coming back through Grand Teton on our way to the airport in Jackson was like coming home. If you're going to Yellowstone, make sure you spend at least two or three days in GT, you won't regret it.