National park guides
A few years ago I got tired of having to research from scratch what there is to do at a couple of my favorite national park spots each time we went. So I made a couple quick reference guides with the relevant info. Because I have small kids, these are oriented around lighter hikes and family activities, so day hikes aren't on these. Be warned that they may be a bit out of date, but I expect they're mostly still accurate and at the least they'll be a great starting point for what options are available.
Rocky Mountain National Park & Estes Park
RMNP requires day passes reserved in advance for different sections in the park. It can be tricky to get the passes and there are other cool things to do in the area, like in Estes, so this guide gives a number of options.
Grand Teton & Yellowstone
The big thing to know is if you want a camping spot you'll need to reserve it when they become available 6 months in advance (more for cabins). Otherwise this covers activities and hikes in the Tetons, and a day trip up to Yellowstone.