The Day you reach Manali, you can visit local Manali town area such as Mall Road, Van Vihar, Hadimba Temple, Tibetan monastery which is close to mall road.
On Second Day, you can visit Solang Valley, Atal Tunnel & Sissu Waterfall in one day as in Solang you might take some time to enjoy activities like ATV rides, Paragliding and others.. You can do this trip by Taxi, Bike, Scooty or through govt. transportation from Mall road.
On Third day, You can do Kullu, Manikaran & Kasol Trip in one day from Manali.
If you have more days in Manali, here are some places you should visit which are unique and not much touristy..
In one day, you can visit Sajla Waterfall, Naggar Castle & Museum & Jana Waterfall which is famous for Local Himachali food at Maniram ka Dhaba near the waterfall. Also this entire road looks beautiful on a Bike, Scooty.. You will see lot of Apple Orchards here.. If you have some time you can also visit Soil village on the way.
One Day, you can spend for Hiking to Jogini Waterfalls, Keep in mind the weather should be sunny if you want to see rainbows in waterfall.
One Day you can hike in Old Manali during the day, visit Manu Temple and do some Cafe hopping in the evening, there are lot of cafes in Old Manali with Live Music. You can explore Lazy Dog, Rocky’s Cafe etc.. For Old Manali Tour You can book this trip here.
If you have more days in Manali, You can visit Parashar Lake which is a floating island lake with Pagoda like temple dedicated to Sage Parashar. This is a fresh water lake at an altitude of 2,730 Mtr. You can also visit Kullu on this day for Rafting if you have extra time or Shopping in Kullu for souvenirs.
To Book your Trip to Parashar lake Please visit here.