Beijing's most famous brand of sweetmeat pastries, Daoxiangcun offers ideal rucksack fillers for your hike up Tai Shan. The flaky pastries have candied fruit fillings and are sold in biscuit-sized pieces (you'll get four or five for around ¥10) that you can mix and match.
Ones to look for include zǎoní sū (枣泥酥, jujube cakes), which taste like fig rolls, nánguā bǐng (南瓜饼, pumpkin cakes), and máxiāng sū (麻香酥, sesame paste cakes).