If you travel and want to see a variety of plant life I would recommend:
Dow Gardens, Midland, Michigan
Minnesota Landscape Arboretum, Chaska, MN (near my home)
Arizona-Sonoran Desert Museum, Tucson, AZ
Desert Botanical Garden, Phoenix, AZ
We caught the desert in full bloom in early April in Phoenix and Tucson. Dow Gardens and our own Minnesota Landscape Arboretum are the two best we've been to for trees and flowers.
Our national parks are our national treasures. We travel mostly in the off-season so don't have to put up with the massive summer crowds. Surprisingly there are a lot of national monuments that are actually better than some of our national parks. This trip we searched out some more Ancient Pueblan ruins in Arizona and New Mexico. This fall we are committing ourselves to another Class B RV rally in Texas and then a trip through the Gulf states over to Florida. Hope it is a slow hurricane season, but if so, we can quickly head inland.