It's my father's birthday. He always told me that people remember the first thing you do with them and the last thing.
I don't know about you, but it always seems that I remember the very last thing that I do in practice. For some reason, whatever the last exercise that I play is the one that sticks with me.
For that reason I will make two suggestions, and you can take whichever one you like.
The first, is that you finish the practice with something that is really fun. On occasion I play something that I REALLY like to finish my practice. That way I remember playing something fun. When I am preparing for the next practice, the fun I had at the end of the last practice comes to mind. That puts me into a good mood so that the next practice starts off well.
The second suggestion is that you finish with the piece that you are working on . . . perhaps the part that you are having the most trouble with. The key here, though, is that you play the passage perfectly NO MATTER HOW SLOWLY YOU HAVE TO PLAY IT. The mental image here is that you finish with success. You will start out the next practice session building on the success that you experienced the last time.
Either way, the key is to finish strong so that when you pick up your violin the next time you are building on fun and success. And that is what playing the violin is all about!