How to react when...?

I have been wondering for some time already but can't seem to find an answer to these questions. (Sorry if they've already been answered but I didn't find anything relevant)

How are we supposed to respond upon having a employee in a shop bow to us saying "irashaimase", "Youkoso" or any other kind of greeting?

I was wondering that because the other day I went into a sushi-bar and when the waitress greeted me, I froze without knowing how to react.
Somehow I am glad that when she saw my confusion she switched back to english and asked me how many people I need a table for.

Still, this made me wonder, if I was to go into a department store or something like that...how am I supposed to react?

Thanks in advance for clarifying this.

Look at what the other customers do. Usually nothing.

Nothing, nodding or smilling will do.
If you often go there and you don't want her to switch to English, just say (lightly in a rising tone), "domo". I am 73% sure that she will take you as a Japanese speaker without a I_speak_Japanese T-shirt.

I see...it might seem weird but I should get used to it like...I guess...
Thank you.

The big onsen ryokans really make you feel like a rock-star when they line up in a roll to greet you as you come off the buses.

I used to be uncomfortable about it as well but to Japanese nationals it's something that often times is expected and sometimes they even get mad if they don't get receive this type of service (お客様は神様です).

And it extends to other businesses as well where you are greeted when entering an establishment (bookstores, restaurants, drinking bars, small grocery and convenient stores, banks and even salons/barber shops).

It's something that I definitely miss about Japan, the good customer service and eagerness to greet and welcome patrons/guests. While some businesses in the US like high-end clothing retailers and classy restaurants are starting to do some of the same, it definitely is something you only experience in Japan and maybe even Hong Kong and parts of China.