'I' applies to the speaker only, while 'there is' includes all present as possible laborers. 'Plenty' is more than 'some'. 'To be done' is the passive voice of 'to do'-- perhaps the passive stressing that the tasks exist and the active stressing the action that must be taken. Other than that, I see no real difference in the communication intended-- they all mean that you can't punch out yet.
This is a good English Grammar website. If you want a list of rules, I often try
THIS PLACE.