Lainzer Tiergarten is a former imperial hunting reserve on Vienna's western edge, now a large public nature reserve with woods, meadows, and marked walking routes. Wild boar and deer still live here, which gives the place a much wilder feel than the city's formal parks.
The main cultural stop inside the reserve is Hermesvilla, the country mansion built for Empress Elisabeth. Come for a half-day walk, especially if you want a quieter companion to nearby Schönbrunn.