Exploring Rishikesh in 12 hours is not that hard as it sounds. But there are limited things you can do in 12 hours.
Making a list is very important before you set to explore rishikesh. Specially people travelling from delhi, here is a small tip of what are the things you can do in 12 hours and back home the same day.
Take a night bus from delhi to rishikesh from kashmiri gate. You will probably reach rishikesh Bus Terminal around 2am in the morning. Upon reaching rishikesh, take a share auto and reach Ram Jhula.