If you forgot to say ata chonantanu after Shabbat (or Jewish festivals), you do not need to repeat the amida. But, if you then ate food before saying havdala, you must repeat the amida including ata chonantanu.

On Shabbat, even if there will not be at least nine men answering amen to the reader's repetition of the amida, the reader should say the repetition anyway, without intending that his repetition be counted as a nedava (which can only be offered on weekdays). If less than six men will be answering, the leader must wait before doing the repetition.
 

As for Shabbat, Jewish festivals start for women when they light candles or at sunset, whichever is earlier. 
For more details, see Jewish Festivals: Candles: Lighting with Delay until Sunset.
 

You may wear collar stays on Shabbat even where there is no eruv.
 

If you receive an aliya to the Torah on Shabbat and Jewish festival mornings, it is customary give a donation to the synagogue, but you are not required to do so by halacha (and you may not give the donation ON Shabbat!).

Inviting friends to your house for meals, even on Shabbat and Jewish festivals, is not hachnasat orchim--unless the friends do not have food or a place to sleep. But inviting friends for meals may qualify as other mitzvot.

Dry food that has been fully cooked before Shabbat may be heated in an oven if the oven is off even if the oven is hot (120° F--49° C--or more).

If you do not normally carry on Shabbat even in an area with an eruv, you may not ask another Jew to carry something for you.  But if the other Jew does carry an item, you may use it.
 

You may not wear a non-decorative watch on Shabbat where there is no eruv or if you have the custom of not relying on eruvs. If you would wear your watch as a piece of decorative jewelry even if it did not work, you may wear it on Shabbat even without an eruv.

Before eating bread (at any time, not just on Shabbat or Jewish festivals), dip the bread in some salt.

If there is not enough wine or grape juice for Shabbat (or Jewish festival kiddush) and havdala:

• Set aside the first cup for havdala; then, if there is one more cup,
• Use it for the morning kiddush.
• See How To Do Shabbat Daytime Kiddush and How To Do Shabbat Evening Kiddush .

 

You may not tie permanent knots on Jewish festivals (and Shabbat).

Before eating bread (at any time, not just on Shabbat or Jewish festivals), the bread should be dipped in some salt. This makes it taste better and makes it like a sacrifice (which had salt added to it).

You may not light Jewish festival (or Shabbat) candles before plag ha'mincha.  The candles must burn until at least dark (tzeit ha'kochavim) and someone must be there to see the light from the candles after dark.
SITUATION  Mincha minyan begins at plag ha'mincha. You cannot light candles at home and still get to mincha minyan on time.
WHAT TO DO You may light a candle without a blessing, just so you can have a flame for after the festival has begun. You go to synagogue and after ma'ariv you return home and light the candles from the flame which was burning from before sunset. If you will not have a flame burning from before sunset, you must say mincha on your own (anytime from half an hour after mid-day until sunset). You will light candles after plag ha'mincha but before sunset and not join the mincha minyan. Women should skip mincha and light candles either 18 minutes before sunset or have a flame burning from before sunset and light candles from that flame once the festival has begun (but she may not light from a new flame or a match).