The house sitting assignment should end in the same spirit that it began: with an exchange of clearly expressed needs and plenty of good will. Don't surprise your house sitter by arriving home at any time other than the one you've told them about. They may have left a lot of the necessary preparation for your return to the last minute or they may have arranged their transport out of town very soon after your purported return (potentially leaving your animals in the lurch).
If your house sitter has paid you a security deposit they may be keen to finalise the financial aspect of the assignment before they leave. This is a fair enough request, even if all you want to do is lie on your own sofa again and enjoy the quiet peace of returning home (bliss!).
Be prepared to go through some of the details of the assignment with your house sitter to work out who owes who for what. Have the security deposit refund available by cash or checque. Above all, before you get home, tell your house sitter how you'd like to see the assignment end (you may not want them in the house when you return). They're used to being flexible and meeting your needs.