The experience of traveling with Celestia would entirely depend on how self-sufficient she is. She's had so little focus, so we don't really know how her daily life is. A ruler such as herself probably has a lot done for them, which they wouldn't have on a road trip. On the other hand, her magic may be able to literally God-Mode her way through just about anything, and she could very easily be a do-it-yourself type of pony. So, yeah, it pretty much hinges on that.
I get the feeling that Pinkie would go mad with power, prompting the other cast members to stage a coup. It quickly becomes a parody of that sort of story. And of course no one else in Equestria notices what's going on.
Pinkie would install trapdoors all over the castle, and immediately drop anyone who so much as looks at her the wrong way down one in classic super-villain fashion.
Mainly, I'd want to travel with the pony who I could strike a good conversation with. I think Celestia would be good in that regard. She has a thousand years worth of stories to tell, and maybe... JUST maybe, I could ask her about Luna, and Nightmare Moon and such
"Schneizel! So you ARE the
puppet master behind all this!" - Lelouch Lamperouge
Tealove is best pony
Traveling with Princess Luna would be quite an experience. I mean, we could learn so much from each other. And I'm sure she'd love to have a traveling companion to enjoy her night with her.
It'd be brutal yeah, but it'd also be adorable too.