There are few sport activities in Manali itself,
Cycling/Biking : Motorbikes/Cycles are available on rent
River Crossing : In Old Manali there are couple of river crossing opportunities. For couple of hundred rupees you can do this. (that is what I am doing in the pic above)
Trekking : There is a Himalayan Trekking group in the main market which takes daily treks which you can join. They also customize the treks for a group if required. Other tourist guys will also take you trekking.
Apart from these adventures that are in Manali itself, there are a lot more activities to be done around Manali.
White Water Rafting : An hour or so from Manali are the Beas river rafting groups. You can go on 5/7/9 km rides. Its a lot of fun!
Paragliding/Skiing : At Solang Valley which is again just a few hours from Manali - you can do paragliding, skiing, ropeway ride, zorbing etc.. Its quite an adventure spot.
I am sure there are still more adventure sports like rock climbing that would be available in this region for a real adventure buff! If you go later in the year post May the higher altitude places like Rohtang/Chandrataal would be open. There would be lot of fun activities there as well :)
Also, you can select from various Manali tour packages available, as these packages often include adventurous activities as well.
There are numerous sport activities that you can enjoy in Manali. The beautiful place offers a myriad of opportunities for different kind of sporting activities where you can indulge yourself. For all the information on the super fun, adventure sports like Paragliding, Rock Climbing and River rafting you can check the things to do section of our Manali travel guide page.