Since most of the museums in the city are government-funded, you can just walk in and take a look, though some of these do have nominal ticket charges. However, entry to Gandhi Smriti, National Police Museum (closest metro station is Central Secretariat), the Philatelic Museum (Patel Chowk metro station), and Airforce Museum (Sadar Bazaar Cantonment Metro Station on Magenta line), to name a few, is free.
Timings:
Gandhi Smriti, National Police Museum: 9:30am to 5:30pm
Airforce Museum: 10am to 5pm