Practicing on a long lead in Parks is a GREAT idea. New territory, lots of other distractions etc. I would start either lightly burying them, or just obscuring them- depends on the levels of distraction present. Also if there are trails in the park have her travel on the path for a while before you plant a target so she gets used to not finding them right away. That is a key skill that is often overlooked.
As per the video:
Her persistence is fantastic! Love that. At the 1:43 mark- wow. Love it. You guys are doing great! K eep working on her re-alerts, it will be helpful later on, but wow- great job.
That was a great session. Good job you two.