Sweet Caroline. Additional feel-good due to it being a Julia Nunes cover.
Cabaret!!
The first song that came to mind was Walking on Sunshine by Katrina and the Waves.
Some of the other songs I go to to dump the blues:
All Summer Long - Kid Rock
Jump - Van Halen
Almost Home - Mary Chapin Carpenter
Get On Your Feet - Gloria Estefan
Down at the Twist and Shout - Mary Chapin Carpenter
Jump - Pointer Sisters (Check out Hugh Grant dancing to this in Love Actually)
Navajo Rug - Jerry Jeff Walker
Keep Me In Mind - Zac Brown Band
Boogie Woogie Bugle Boy - Bette Midler
Footloose - Kenny Loggins
Let It Roll - Little Feat
Get Over It - Eagles
What A Wonferful World - Louis Armstrong
All She Wants To Do Is Dance - Don Henley
She Thinks My Tractor’s Sexy - Kenny Chesney
Different Kind Of Fine - Zac Brown Band
Summer of '69 - Bryan Adams
Rock and Roll All night - Kiss
Boogie Woogie Stomp - The Majestic Soundtrack (Jim Cox)
Accidentally In Love - Counting Crows
Jump, Jive, & Wail - Brian Setzer Orchestra
Who Says You Can’t Go Home - Bon Jovi (with Jennifer Nettles)
Centerfield - John Fogarty
Everybody Wants To Go To Heaven - Kenny Chesney
Moondance - Van Morrison
Scarlet Begonias - Grateful Dead
Werewolves of London - Warren Zevon
There are so many others. YMMV
Mack the Knife!
9:06 am real oldies, spirit in the sky
So many good ones listed!
A couple of the previous songs, I knew as the Jimmy Buffett covers. 
Here’s a couple “out of left field” -
Tom Chapin (brother of Harry) does kid songs. He’s as cheerful as Harry is melancholy. 
These two, I liked so well they’re on my playlist. (Talk about yer guilty pleasures!)
“Family Tree” It explores the concept (amusingly) that if you go far enough back, everyone’s related.
♫ Before the days of Jello, lived a prehistoric fellow,
Who loved a maid and courted her beneath the banyan tree … ♫
“What does a Didgeridoo Do?” (Not on Youtube that I can find.) Witty limerick verses about musical instruments with a chorus that describes a didgeridoo.
♫ In Hawaii I learned ukelele,
From a young hula dancer named Hayley.
She said “I like to play
As I practice my sway …
So, I ukelele daily!” ♫
The Association
First one that popped into my head was “Wake Me Up Before You Go Go” Wham!. Second one to pop into my head was “Don’t Worry Be Happy,” but that’s already been mentioned.
DJ Jazzy Jeff & The Fresh Prince - Summertime (Especially on the first day of summer)
Ramones - Hey Ho
Kool & The Gang - Hollywood Swinging
70’s AM Radio is often considered a stain on humanity, but I love it:
I Wanna Kiss You All Over - Exile
I Love You - Climax Blues Band (love the line, “I was playing around, feeling down, hitting the beer”… hitting the beer?!?)
Rock Me Gently - Andy Kim
Life is a Rock - Reunion
Midnight at the Oasis - Maria Muldaur
Come on Get Happy - The Partridges
Undercover Angel - Alan O’Day
Arizona - Marc Lindsay
Some good feelin’ Disco:
Born to be Alive - Patrick Hernandez
Rubberband Man - The Spinners
We Are Family - Sister Sledge
You Sexy Thing - Hot Chocolate
YMCA - The Village People (great video!)
We’ll have to go to the videofor an expert’s opinion on the most inspiring Florence and the Machine song.