It is possible, upon availability, to book activities on the spot during the festival. However due to limited capacity, some activities or slots for those activities might be sold out by then.
Next to the activities offered in the Add-On Sale, there are various activities in Alpe d’Huez like an ice skating rink, swimming pool, wellness, etc. that you don’t need to reserve up front. Explore the city of Alpe d’Huez on their website: https://www.alpedhuez.com/en/winter/home/