Taxi or subway from temple of heaven to summer palace?

If taking subway, the train ride is about 50 mins, and in addtion to the time for transfer and waiting, you may need another 20 mins. So the time is about 70 mins in total. The detailed transfer is like this: Take the subway line 5 from Tiantan Dongmen to Chongwenmen Station, where you will transfer to line 2 to Xizhimen Station. Then transfer to line 4 to Beigongmen. The fare is CNY6.

If taking a taxi, it takes about 35 mins and the fare is about CNY85.

Thank you for your reply. Taxi sounds good if we run out of time. ;)

By the way, do you know any nice and reasonable price restaurant (Chinese or other Asian type) nearby summer palace or Tiantandongmen subway station?

Near the Temple of Heaven (Tiantandongmen metro station), there are several great Chinese restaurants for your choice. They are Dong Lai Shun at no. 91, Tiantan Road, Jin Shi Yuan Noodles with Soybean Paste at the intersection of Tiantan Road and Qinian Street and Imperial Cuisine Restaurantat close to Jin Shi Yuan.

For the imperial garden, several good restaurants are available in the park. You can find fast food at Zhichun Pavilion, Paiyun Palace and Shizhang Pavilion. A imperial cuisine resto is at Ting Li Guan.

We change plan a little bit by going to summer palace before zoo. But I didn't fund any info if we can take a boat to the zoo from the palace? If yes, how much?

Well, you can take the boat to get to the Zoo directly.
As I know, the fare is about CNY 40 for the single trip.