I have but one day in Bryce in mid July. Plan to do the 2hr. horseback trip to the floor of the canyon. Can anyone give some advice as to another Bryce activity (possibly a short hike) before we head to Torrey and Canyonlands? Thanks.

What kind of physical condition are you in?
The trip to Tower Bridge is fun, but coming back is uphill all the way, at 8,000 feet.

I'm traveling w/ my 20 & 23 yr. old kids - we're in good shape but my 76 yr old mom definitely couldn't do that!! - though I guess we could leave her behind Is the Navajo Loop strenous?

Let me also mention the heat. Take LOTS of water. And just when you think you have enough water, take more.

You could spend some time to drive south in the park to Bryce Point. There are some short walks on the rim and I think the views are better than anywhere else.

Might also try a viewpoint called Natural Bridge. It is just a small overlook that shows a neat Arch very close to the parking lot. I am sure Mom could do both of these places.

From Sunset Point, there is a trail that takes you to the bottom in a hurry and circles back up the same way. I ran out of time to do that but wish I would have. This would not be for Mom. We are from the Chicago area and with Bryce being above 8000 ft., we noticed a shortness of breath on the inclines in a hurry. You just need to take your time unless of course you are acclimated to elevation.

We did the Navajo Loop/Queens Garden hike last May. I wouldn't call it very difficult, but it could be too much for mom. She could probably try it part way. It's a great hike, however.

Drive to the end of the park. There are many lookouts where you can take great photos.

anything below the rim requires a steep hike coming out.
What a young person can do with ease, an old person who does not exercise regularly would find very taxing.
The altitude is enough to be a hazard to anyone with cardio vascular problems.

travelbug ~ Try the Bristlecone Pine Loop trail.
This is a nice, easy trail w/look-out views that are breathtaking.
When you get to the 1/2 way mark, there is a bristlecone pine tree that is supposedly the oldest living tree @Bryce ~ more than 1,000 years old.
Some of my best pictures came from stopping along this trail while @Bryce.
Have a great time!

The Navajo Loop/Queen's Garden is a nice combination. I doubt that your mom could do it, but it's not particularly strenuous if you've acclimated to the altitude. The switchbacks coming down the Navajo Loop are impressive - walk down these switchbacks and up the Queens Garden trail if you do this loop.

If you've already ridden down into the canyon on a horse, you may not be in a big hurry to walk right back down. The scenic drive on the rim has several viewpoints, some of which require short walks. That might be enough for you. The Bristlecone Loop is a little boring till you get out to the viewpoint, but it's probably a good choice if everyone wants to go at once.