The sentences have been successfully translated into Spanish.
After arriving at the youth hostel, we went to the reception. → Después de llegar al albergue juvenil, fuimos a la recepción.
On entering the room, we saw that the floor was dirty. → Al entrar en la habitación, vimos que el suelo estaba sucio.
Before eating, I'm going for a swim. → Antes de comer, voy a nadar.
After arriving at the beach, I went for a walk. → Después de llegar a la playa, fui a dar un paseo.