I just noticed that no one has replied yet, so I thought I'd say hello, and explain that you may not get many responses here (though I hope I'm wrong.) It's not that we're ignoring you, it's just that most of us don't have much experience with the type of pack you're looking for. Most of us are centered on wilderness hiking and camping, and tend to cluster toward the lightweight end of that spectrum. (Personally, it's been years since I've used a pack larger than 3500 cubic inches, let along one that has a detachable day pack.) It seems, from your description, that you're more looking for a travel pack, and that weight may not be a problem since you won't actually be hiking with it. (From the general tone of your post, I'm thinking maybe from the terminal to the cab, and the cab to the hotel room?) If my read is accurate, most folks won't respond simply because they don't have any useful information for you.

About all I can do is suggest that, in general, Osprey, Deuter, Gregory, REI and Jansport are reputable brands, and I think they all have larger packs that have the features you're looking for, or perhaps a system of clips or attachment points that let you add a smaller pack to it (it may not be sold as a single unit, so you can choose which model of smaller pack you want.)

I'm sorry I couldn't be more helpful.