Noi Due. You can take non-Jewish or non-kosher guests and nothing will feel "off" to them. Prices are lowere because it's Italian/dairy. But the food is wonderful. http://www.noiduecafe.com/
Atmosphere is upscale/casual. If they're visiting from out of town, the Columbus circle, Lincoln Center location is a special advantage.
Meat restaurants in Manhattan are generally either very pricey or casual/embarrassing. You didn't specify how casual, but there are delis, Mendy's and Second Avenue. Here your standards fo kashruth come in. Many people trust Second Avenue because they trust the (too eccentric to close on Shabbat) family that runs it; many don't .
On that same theme, there are very popular vegetarian Indian restaurants (very popular among honest-to-gosh Indians) that are supervised and open on Shabbat. You get authentic, interesting food for an amazingly low price but, there is the question of supervision.
Your other option are the two Indian kosher places (closed on Shabbat). Dakshin and Shalom Bombay. http://chowhound.chow.com/topics/772329 I like them both very much. Prices are lower because than Colbeh or Abigael's because the use of meat is more economical. They are non-embarrassing because you are getting a completely normal ethnic restaurant atmosphere.
We have definitely considered Noi Due, which we like very much, and would be perfect, except for the fact that, as I remember it, it's noisy, and I don't think it's conducive to conversation with relatives that we haven't seen in a long time. Do you think that I'm wrong about this?
As for the other suggestions:
At Le Marais, we can't realisticly limit our guests menu selection to the more reasonably priced menu items. Ditto for Abigael's.
Pita off the Corner sounds too casual. We do want table service.
Can anyone give me more details about the suggestion of Dakshiin II, which I've never heard of. (Is this Indian?), and Colbeh (which I think is Persian?) as far as price, type of menu, ambience, etc.
As for deli's I'm not a big fan of Mendy's. We'd probably all enjoy the food at Second Avenue, but it falls below our Kashrut standards.
Any more feedback and new suggestions would really be appreciated.
Colbeh is slightly less pricey than Solo, and the non-frum find the sushi embarassing/amusing in a Persian/Mediterranean restaurant.
Noi Due is not noisy when it is not crowded, which may or may not be a useful piece of information. You might ring them up and ask them how crowded it usually is at the time you would prefer to go.
Shalom Bombay and Dakshin look like what they are, neighborhood-restaurant level Indian places. Tablecloths. Dakshin has a more modern look - exposed brick walls. Both can be noisy when crowded.
Levana's with the fabric walls used to be quiet. Is there a quiet kosher restaurant in Manattan?
I just got back from New York, and I and I ate at Noi Due 3 nights in a row. On one of the nights, there were six of us at a round table in the corner. There was a lot of background noise, but we were able to hear each other pretty well probably because the table was round and small. The food was fabulous. I would highly recommend the jalapeno tuna burger, the lasagna, and the sweet potato ravioli. I would definitely call ahead and see how long the wait is for a table - we waited for close to an hour on one of the nights.
Also, how about Noah's Ark instead of 2nd Avenue Deli?
Are the relatives into dining out or more of the Olive Garden variety? I don't mean to sound snobby but it makes a difference. I'd go with Noi Due. It has great food and it will feel like a nice NY bistro.
I wouldn't take non-kosher (or vegetarian) guests to Abigaels. It feels like it's trying to be a steak house but it doesn't quite work out that way. Maybe it's that they serve sushi but aren't Japanese.
I suppose it's a question for another thread but can someone explain why almost every kosher restaurant in the tri-state area feels compelled to include sushi on their menu despite the focus of the menu and the quality of the sushi?