in Paris...

It's late spring in Paris now, the daytime is a bit longer than Hong Kong. Usually sunrise will be around 7am and it will go down around 8pm at night. But in June, you will have the longest day, Sun will be on the sky from around 6:30am to 9:30pm...
Actually both cash and credit cards will be accepted in brand shops and department store. French, same as HK people, will use credit card (they call it "carte bleue"). I don't believe someone will refuse to accept a cash payment !! It's legal tender, they cannot refuse to accept. The method of payment doesn't affect the TVA refund. If you buy things in department store over the limit, you can ask for TVA refund at once in the service counter.
I will recommend a "common" restaurant in Chamy-Elysees, just next to the main store of LV. It's called "Leon de Bruxelles". It's a special restaurant for mussels. They can be cooked in different ways. The price is quite good, only 25 euro you can have a set dinner with a big pot of mussels, fries, desserts and unlimited bread.
There are so many supermarkets in Paris, same as in Hong Kong. Usually I went to Franprix, Ed Epicier or Leader Price for shopping foods and groceries. You can ask the staff in hotel / hostel, they will show you the way to the supermarket. Don't worry, many many shops
I always believe the prettiest section of the "quai de Seine" is from Notre Dame Cathedral to Pont Alexander III. I would recommended to to spare 30 - 45 minutes for walking along the quai. Please walk along the river on the left bank (that means lower part of the map), starting from Notre Dame towards Pont Alexander III, and then you can have a very very nice promanade. I believe you will love Paris after this walk.