How to plan for our sprint right, so we can deliver our sprint with all agreed stories and also what if we finished all the stories assigned with this sprint, but the sprint isn't finished yet, what shall we do?, do we finish the sprint early?, do we take stories from the product backlog?, do we take a break?
In these case, I would usually take additional stories from product backlog. However, this is based on several factors (e.g. How ready are the stories? What is the motivation level of the team to take on additional stories?)
I would suggest that you discuss this with the team, esp tech lead and product owner to get their feedback. Sometimes, it may be necessary to spend the time to clear technical debt / increase test coverage etc.