When adding custom contact activity, you can also specify descriptor template. Specified template will be used to generate descriptor shown in contact's profile timeline to provide additional details for activity. You can use liquid templating to render output. Activity record properties can be used as variables. For example, below is an example of rendered descriptor for Review Added activity.
Here's the descriptor template used for Review Added activity.
Title: {{ review_title }}
Content: {{ review_content }}
Rating: {{ rating }}/5
Product: {{ product.name }}
To view available variables for Review Added, see it's resource properties page and resource records page. Using the same concept, you can specify your own descriptor template for custom activity.