One thing that's really added value to my (so far almost) 30-day, 4000-mile trip has been the National Park Service App. It allows online or offline access to information about hundreds of national parks, historic sites, and monuments across the country and in US territories. I've especially enjoyed the information about hiking trails and driving routes.
My favorite capability, though, is that it allows you to keep a list of those places you've visited! On this trip I've added Montezuma's Castle, Petrified Forest/Painted Desert, Petroglyphs, Florissant Fossil Beds, and Devils Tower to my list of National Park Service sites I've visited, and tomorrow I'm going to add Golden Spike National Historic Site to the list (being from the Sacramento area, that one holds special significance to me). It will be my 37th.
I encourage you to download this app, mark those sites you've visited, and start planning on more!