The Lovely Spreewald:
https://www.getyourguide.com/berlin-l17/from-berlin-bus-tour-to-burg-in-spreewald-with-boat-tour-t29813/
https://www.berlin.de/special/reise/brandenburg/spreewald/
http://www.berlin-tour-and-guide.de/en/full-day-tours/spreewald-sightseeing-with-boat-tour/
http://www.spreewald-experience.de/en/getting-there/
https://www.grosser-kahnhafen.de/service/gutscheine

Free or Very Cheap:
https://originalberlintours.com/tours/free-original-berlin-walking-tour/
http://www.neweuropetours.eu/berlin/en/home?&Tour=419&date=07/07/2017
http://www.neweuropetours.eu/berlin/en/home
https://www.google.de/maps/place/Tierpark+Berlin/@52.5023038,13.5291672,17z/data=!3m1!4b1!4m5!3m4!1s0x47a84936a676bfdd:0xbfb429b32db4ffdc!8m2!3d52.5023038!4d13.5313559
http://www.sammlung-hoffmann.de/

Not Free, but Good Value for the Price:
https://www.tripadvisor.com/AttractionProductDetail-g187323-d11453420-Berlin_Off_the_Beaten_Path_Walking_Tour_Neighborhoods_of_Kreuzberg_Mitte_and_Friedrichshain-Berlin.html
http://alternativeberlin.com/tours/berlin-graffiti-workshop-and-street-art-tour/
http://www.insidertour.com/tours.php/cat/27/id/40/title/The_Famous_Insider_Walk
http://www.insidertour.com/tours.php/cat/27/id/42/title/Cold_War_Berlin
http://www.insidertour.com/tours.php/cat/27/id/78/title/Berlin_Today
https://therme.bad-saarow.de/info-service/unsere-preis%C3%BCbersicht-f%C3%BCr-ihren-wellnessurlaub

Potsdam:
https://www.likealocalguide.com/berlin/tours/best-of-potsdam-day-tour-from-berlin?utm_campaign=marketplace_click&utm_medium=tours_page_big_image&utm_source=berlin&utm_content=tours
http://www.insidertour.com/tours.php/cat/27/id/41/title/Insider's_Potsdam

Distant Past:
http://www.insidertour.com/tours.php/cat/27/id/73/title/Jewish_Berlin
http://www.insidertour.com/tours.php/cat/27/id/50/title/Pergamon_&_New_Museum_Tour

Expensive but Might Be Worth It:
http://www.insidertour.com/tours.php/cat/27/id/62/title/Day_Trip:_Berlin_to_Dresden
https://www.viator.com/tours/Berlin/Dresden-Day-Trip-from-Berlin/d488-5560DRESDEN
https://www.tropical-islands.de/preise-buchung/angebote/ticket-angebote/
https://markthalleneun.de/basisangebot/3
http://www.biteberlin.com/

Mum & Sister's faves:
https://originalberlintours.com/tours/the-free-original-berlin-alternative-tour/
http://www.grayline.com/tours/berlin/berlin-most-famous-tour-city-circle-yellow-tour-5881_1/
http://www.neweuropetours.eu/Berlin/en/sandemans-tours/tour-view/FREE-Tour-of-Berlin?date=07/07/2017

If You Get a Welcome Card (A Card You Get From Train Stations, Can Have A Duration Of 2-7 Days, Grants You A Discount or Free Entry to Museums and Stuff)
https://buchen.visitberlin.de/VisitBerlin/offer/detail/BTM00020100381039318?tt=5gs5kuhiho8rj2givsegkimbu1
https://buchen.visitberlin.de/VisitBerlin/offer/detail/BTM00020100422447456
https://buchen.visitberlin.de/VisitBerlin/offer/detail/BTM00020100337359279
https://buchen.visitberlin.de/VisitBerlin/offer/detail/BTM00020100241665978
https://buchen.visitberlin.de/VisitBerlin/offer/detail/BTM00020100224327687
https://buchen.visitberlin.de/VisitBerlin/offer/detail/BTM00020100246225068
https://www.visitberlin.de/en/berlin-welcome-card
http://www.eurocheapo.com/blog/berlin-15-free-and-affordable-museums-under-e10.html
http://www.eurocheapo.com/blog/berlin-budget-guide-to-mitte-neighborhood.html

Good for A Sunday:
http://www.berlinforallthefamily.com/activities/cafeinabusberlin
http://www.berlinforallthefamily.com/activities/visitbergmannkiezberlin
https://www.jumphouse.de/berlin/oeffnungszeiten/
http://www.berlinforallthefamily.com/activities/natural-history-museum-berlin
http://www.berlinforallthefamily.com/activities/labyrinthkindermuseum
http://www.berlinforallthefamily.com/activities/fez-wuhlheide-park-review
http://www.berlinforallthefamily.com/activities/wheretogobowlingwithkidsberlin
http://www.tobewelt.de/preise-infos.html
http://www.berlinforallthefamily.com/activities/sciencecenterspectrum
https://www.aquarium-berlin.de/en
https://dimensionalley.com/
http://www.berlinforallthefamily.com/activities/familyfriendlyswimmingpoolsberlin
http://www.berlinforallthefamily.com/activities/loxxminiaturweltenberlin
https://www.visitberlin.de/en/filmpark-babelsberg
https://www.visitberlin.de/en/madame-tussauds-berlin
https://www.visitberlin.de/en/modellpark-berlin-brandenburg-0
https://www.visitberlin.de/en/waldhochseilgarten-jungfernheide
http://www.top10berlin.de/en/cat/leisure-258/tips-against-boring-sunday-afternoons-2391/german-spy-museum-berlin-4145#1
http://www.top10berlin.de/en/cat/family-276/activities-and-excursions-children-and-families-berlin-3005/europe-raceway-berlin#1
https://www.tripsavvy.com/boat-tours-of-berlin-1520188
https://www.visitberlin.de/en/pfaueninsel-and-pfaueninsel-palace
http://www.topographie.de/en/
https://www.berlinischegalerie.de/en/home/