HOHO - Hop On Hop Off - bus tour is a the best way to 'cover' Delhi in 1 day.
Keep a full day in hand ( at least 6 hours). This is hassle free way to explore the city.
The limitation is the whirlwind nature of a bus tour.
India has more than 200 wildlife sanctuaries and about 100 of them are classified as National Parks. Here is a list of national parks of India. The names of the national parks are organized according to the state in which they belong to. Some of the national parks are speared over many states and only one state in listed in such cases. You can see the same list of national parks in India here arranged in the alphabetical order of their names.