Final answer:
Commas should be placed before and after the phrase 'the tallest mountain in North America' as it is a non-essential clause.
Step-by-step explanation:
Commas should be placed before and after the phrase 'the tallest mountain in North America' as it is a non-essential clause. The revised sentence would be: 'They went to Denali, the tallest mountain in North America, on their first day in Alaska.'