Day one: get them to know the team, who is an expert in what, who works/worked on what, etc. (group lunch is awesome on day 1, I would also recommend 1:1 lunches with engineers and other disciplines over the next week or two, if your team always gets lunch together, then a coffee in the afternoon 1:1 to get to know others, especially introverts)
Assign an engineer as the go to person during onboarding phase so the new hire doesn't bother everyone (they don't have to be the best engineer who knows all the ins/outs, you can do round robin or play a game: last hire onboards the new hire or last person that breaks production is responsible for onboarding ) . The first task is to set up their computer with software needs, sync the code, load test db, get to know staging environment, get to know the process and be able to run the project locally following an onboarding document. If they hit issues then their quest is to find a solution, update the docs, move to next step. Once the environment is setup, ask them to identify a bug and fix it (if your backlog is full of bugs then assign a list of bugs with priority, junior devs might tackle less complicated bugs whereas more senior can go after more challenging issues), they should go through a code review, address feedback, write a test for it and If it's approved then it's checked in, deployed to staging, then it will make out to production in the next deployment
Engineers are like cats (think of a complicated version of a cat), day one is all about settling in to their new home, some take longer to adjust, some quickly adjust (you got the house cat vs outdoor cat and depends on the breed), pushing them in similar ways that might motivate a sales person or other disciplines doesn't work the same.