Can I combine the Lütschine rafting with another activity on the same day?

Yes, and it works well. An early departure finishes around midday, leaving the afternoon free. Tandem paragliding from Interlaken is a natural pairing: after spending the morning in the water, you spend the afternoon above the valley with views of the Eiger, Mönch, Jungfrau, and Lake Brienz. Two completely different ways to experience the same landscape in a single day.