I've done multiple 7-8 hour trips to my mom's, always just me and my almost 3 year old (started when she was barely 2). I try to leave about an hour or so before nap time. That way she can get settled in, play with some toys or look at books, and then hopefully fall asleep for an hour or two.
We always bring library books (she loves books), a tablet from the library as a last resort, lots of snacks, good playlists on Pandora, and a few new toys from the dollar store. Also, those water coloring books from Melissa and Doug are fabulous!