Midagahara (弥陀ヶ原, Midagahara) is an extremely beautiful place through all seasons of the year, and is a great and easy place to take a short hike into the wetlands. Do note that hiking routes are closed when there is snowfall.
There are a two popular courses available that start at the Tateyama Highland Bus bus stop, and goes along wooden boardwalks in a loop into the wetlands and back. Both of these courses are relatively easy, and feature few steep inclines. A third course goes up to the Tateyama Caldera Observation Deck that features much more of a steep incline.
(Image credit to Midagahara Hotel)