The Delhi Metro is the single most important piece of infrastructure for visitors. It is modern, air-conditioned, extensive, and cheap — it connects the airport, both major railway stations, Old Delhi, New Delhi, and most of the main tourist areas. Buy a tourist card for unlimited travel; it pays for itself within a few rides. The Metro runs from around 6am to 11pm. Download the DMRC app or use Google Maps to plan routes.
Auto-rickshaws and cycle-rickshaws are useful for shorter distances and navigating the narrower streets of Old Delhi that the Metro doesn't reach. Agree on a fare before you get in — meters exist but are rarely used. App-based ride services (Ola and Uber) work well in Delhi and are generally more reliable than haggling with rick-shaw drivers. Taxis from the airport should be booked at the prepaid taxi counter inside the terminal to avoid overcharging.
Walking works well within neighbourhoods — Khan Market to Lodhi Gardens, or Chandni Chowk to the Red Fort — but the distances between Delhi's scattered sights make a car or Metro essential for anything further. The heat (April through June) and air quality (November through January) make long outdoor walks genuinely unpleasant in those months.