For Apartments, low cost options include among others:
Replacing CFL and incandescent bulbs with LEDs. Light bulbs are used every day of the year so the savings are pretty significant. LED prices are cost effective and the small premium paid to obtain a LED is recovered within a year! Link to Lighting Calculator
Replacing an old ceiling fan with a BEE * star rated ceiling fan. This appliance is used almost every day. The savings within a year pay for the additional cost incurred in the purchase of the ceiling fan. Link to Ceiling Fan Calculator
Using khus curtains to keep the rooms cooler in summer.