Neelgarh waterfall for trek is good and for sightseeing laxman jhula in night is amazing also must go Neelkanth Mahadev temple and Little Buddha Cafe for Mexican dishes.
If you're in rishikesh don't miss out on rafting, especially the longest one they call it marine drive. Answer to your question there are several treks you can choose from: Roopkund Trek, Valley of Flowers Trek, Nag Tibba Trek, Deoria Chandrashila Trek, Kuari Pass Trek, Kunjapuri Temple Trek Do check out the rishikesh photo blog, in my profile.