Is it necessary for an American citizen with an expired passport to
have a new passport to enter Canada for a few days visit?

Hi maryfrancis,

No, U.S. citizens are not required to carry a passport to enter
Canada, however, you will need proof of citizenship and a photo i.d.. 
Your expired passport may be used as proof of your U.S. citizenship
(it wouldn't hurt to print out a copy of my second link and take it
with you because everything will be at the discretion of the border
official at the time).

U.S. Department of State
Tip for Travelers to Canada
"Current Requirements for Entry Into Canada
Visas are not required for U.S. citizens entering Canada from the U.S.
You will, however, need:
   1. Proof of your U.S. citizenship such as your U.S. passport ..."
   2. photo identification, such as a current, valid driver?s license.
http://travel.state.gov/travel/tips/regional/regional_1170.html

U.S. Department of State
"PASSPORTS: U.S. citizens who travel to a country where a valid U.S.
passport is not required will need documentary evidence of their U.S.
citizenship and identity. Proof of U.S. citizenship includes an
expired U.S. passport, a certified (original) birth certificate,
Certificate of Naturalization, Certificate of Citizenship, or Report
of Birth Abroad of a Citizen of the United States. To prove identity,
a valid driver's license or government identification card are
acceptable provided they identify you by physical description or
photograph..."
http://travel.state.gov/travel/tips/brochures/brochures_1229.html

"The On-Line Office" Services Provided by Congressman Murtha
Passports for Travel Abroad
"The best document to prove your U.S. citizenship is a valid U.S.
Passport. Other documents that establish U.S. citizenship include an
expired U.S. Passport, a certified copy of your birth certificate, a
Certificate of Naturalization, a Certificate of Citizenship, or a
Report of Birth Abroad of a U.S. citizen. To prove your identity,
either a valid driver's license or a government identification card
that includes a photo or physical description is acceptable."
http://www.house.gov/murtha/services/passport.htm

TOURIST TRAVEL
    * Proof of U.S. Citizenship Required
    * Tickets and Documents for return or onward travel
    * No Visa Required for stay up to three months
    * Vaccinations - None Required
   Usually Acceptable Proofs of U.S. Citizenship:
    * Valid U.S. Passport
    * One of the following carried together with Official Photo ID:
(Photo ID not applicable for minors up to 16 years of age)
    * Certified Copy of Birth Certificate
    * Certificate of Citizenship
    * Certificate of Naturalization
    * Consular Report of Birth Abroad of a U.S. Citizen
    * Note: A Voter's Registration Card does NOT constitute proof of
U.S. Citizenship. Acceptability should be checked with the airline
carrier. Persons using a Voter's Registration Card may be subject to
extensive questioning on return to the U.S by Immigration Authorities.
http://www.traveldocs.com/ca/er.htm

Traveling to Canada
U.S. Citizen
A U.S. citizen will be required by the Government of Canada to prove
that he/she is a citizen of the United States. "This proof may be a
birth record, baptismal record, passport, Certificate of Citizenship,
or Certificate of Naturalization. In some cases, U.S. citizens will be
allowed to enter Canada with just another photo identification
document, like a drivers license, because the officer believes an oral
declaration of U.S. citizenship. A voter registration is NOT official
proof of U.S. citizenship."
http://www.cbp.gov/xp/cgov/travel/vacation/us_citizens/travel_to_canada.xml

I was glad to find this for you. If you have any questions, please
post a clarification request and wait for me to respond before
closing/rating my answer.

Thank you,
hummer

Google Search Terms Used: "expired u.s. passport" canada "proof of u.s. citizenship"

Thank you. This saved me quite a bit of money as I do not intend to
renew my passport again.