Lower-priority backlog items. TODOs atop your new PRs. Code reviews. Exploratory work where you can discuss a design sync and then dispatch one or more agents to prototype async. Any workstream where you can define a loop and let the agent hill-climb towards the goal.

A lot of this is personal taste but the general thing I get most value from is asking an agent to speculatively build every idea, instead of writing down ideas in some backlog for later (it never happens later).