we are staying at the Mercure Beijing Downtown. How easy will it be to get to the zoo from there?

You can first walk about 15 minutes to the Nine Dragoons Mountain metro station where you need to take the metro line 7 and get off at Caishikou station where you transfer to metro line 4 Daxing Line and get off at the Zoo station. It may take about one hours and CNY5 per person.