BTilke

I strongly support the idea of packing a smaller bag with wheels inside a larger one. Check them both (as one piece) on the way over; check just the larger one on the way back. Make sure the smaller one also can be carried easily by hand as it's hard sometimes to manage two rolling bags.
If you're planning to travel a lot by train and want to go second class, the rolling duffle bags are really useful. They're narrow enough to roll down the train aisles without banging into the seats (or passengers). I saw a good selection of them at a TJ Maxx a few months ago.
Don't take just one small bag since you do plan to do some shopping.

suze

Like Beatchick, I've also purchased fun souvenier type tote bags if my suitcase gets too full.

WallyKringen

I like to do things the uncomplicated way. A roll-away that is no bigger than what is (in the US) carry-on legal, and a smallish backpack that I can carry turned around on my chest when in a crowd where I can't trust whoever is behind me (not all backpack designs let you do that, some are chin-high when turned around so try before you buy).

That's it for the trip, no matter how long - you can always do laundry in the evening and wear fresh clothes in the morning if you buy the kind that washes and dries easily. Take along nylon string to hang things, between door knob and shower faucet or wherever. Any more carrying and the trip becomes a chore.

Take with you from home a roll of the good thick packaging tape so you don't have to run all over looking for some or make do with skimpy stuff. Then take a moment to pack your purchases - ask for a box at a shop or post office, mail the things in the middle of the day when lines are short, and be footlose and fancy-free again.

If you find you can't do that for one reason or another, buy a cheap bag and be done with it - give it to Goodwill when you get home or use it as a gym bag until it falls apart. I still use a gym bag I bought fifteen years ago for 8 bucks to bring home my (too many but I got away with it) Swiss Schnaps bottles wrapped in bubblesheets the store provided.
Couldn't really mail them...

Cheers

WK

suze

Along those lines, instead you can go to the post office, purchase a box there and send home your dirty clothes, guidebooks, maps, etc. making room in the suitcase for souveniers.