We recommend exploring the town of Orta San Giulio on your first day. Start by walking around the cobbled streets and then make your way to the lake front where you can catch a boat to the island of San Giulio. Most famous for the Benedictine Monastery, Basilica di San Giulio, there is a path you can follow which will take you all around the island.
The next day take the walk up to the Sacro Monte di Orta which is listed as a UNESCO world heritage site and has stunning views over the western side of the lake.
If you find yourself partial to Italian wine, the Franciacorta area is a must visit whilst you are staying on Lake Iseo. There are plenty of vineyards to choose from in this area where you can sample the local wine. Spend your days either relaxing on the lake front or by the pool, for the more active, rent a bike and cycle along one of the many routes around the lake. There are also many walking tours around the lake ranging across all levels.