Kew Gardens is a well known site in London both for tourists and locals. UNESCO world Heritage site, its a Botanical garden, spread across 300 acres, full of rare plants, trees, a rich bio diversity preserved over centuries.
It will be very ambitious to think about covering the entire park in one day. Its really huge. At the entry you will get a map, you should check and plan your itinerary. There are many areas you can explore, like Royal Pagoda, the Botanical gardens, Prince of Wales Conservatory, Great Broad walk, Palm House. If you want to visit the Kew Palace, you can also plan the same.
There are cafes inside where you can have coffee, brunch, a nice lazy drink near pretty lakes.
Entry to the Gardens is with a ticket. You can buy the same online or at the gate.