Not to be a buzzkill, but are you flying this year and have been in US longer than 72 hours prior to arrival? If so then technically you wouldn't be allowed to do a day trip on your transit.

The Covid rules basically say that you are allowed to transit, but if you aren't just transiting, which existing the secure area isn't, then you need to go directly into quarantine for at least 5 days. Check the Coronavirus Testing at the Airport section of the Frankfurt Airport website.