Sky Garden
You can go on the Eye or up the Shard, but best value is the Sky Garden.
It’s free, but you have to book, and there is security to deal with. But it’s no problem.
On a sunny day, the views are amazing, and you can go out onto the balcony. There’s food and drink available, and a real garden.
Monument station (District and Circle) and a short walk. You can’t miss it!