Green spaces in cities are places where city dwellers can have contact with nature, which, according to the research to date, can have a very beneficial effect on their mental well-being. However, it is still not entirely clear which characteristics of green spaces are most beneficial in terms of their positive impact on people. In this study, we focus on identifying the restorative attributes of tree canopy sites (forests, street greenery) and open green spaces (water, meadows). Four psychometric tests were used to examine the subjects’ reactions before and after exposure to the analyzed environments (the control environment was the room in which the pre-test was conducted). The experiment was conducted with 55 young adult university students. It was shown that, compared to the post-housing (control) environment, all green spaces influenced the subjects’ well-being. The water environment had the most beneficial effect, followed by a meadow, a forest, and a housing estate with the presence of urban greenery. A short walk amid open urban green spaces, which provide more opportunities for observation, has a better impact on the well-being of the respondents compared to tree canopy sites such as forests or street greenery.