Up at The O2 climb is a real time commitment — fit it in only if it's a priority.
Allow 1.5–2 hrs.
The clip-in walk up the springy fabric is a real little adrenaline hit, and the summit view over Canary Wharf and the river — especially at sunset — is fantastic. The honest catch: it's weather-dependent, has age and fitness minimums, and you're paying climb prices for a view the cheaper Greenwich high points partly share.