Beijing Botanical gardens in mid October

I will have a free day in the city while waiting for a connecting international flight. Is October a good time of year to visit it? How long a taxi ride would it be to get there from Capital Airport? Approximately how much would such a taxi ride cost? Do you have other suggestions of what to do with this free day that won't take up a lot of time in transportation? I will need to be back at the airport by 7PM. Thank you.