Golf
Buy some old golf irons on Gumtree ($20) and some plastic balls from Big W ($20) and teach them to hit a golf ball.
Then take them to a driving range:
http://www.thornleighgolfcentre.com.au/driving-range/pricing/
$20 for a bucket of 100 balls, $14 for 50 balls.
Or play Pitch and Putt:
https://www.par3golf.com.au/
$15 for a junior and $25 for an adult.
Mini Golf
https://citygolfgordon.com.au/#prices $13 for children and $17 for adults.
https://www.holeymoley.com.au/locations/chatswood $14 for 9 holes $24 for 18 holes.
Gaming VR Experience
You walk around in a warehouse with a VR helmet on.
https://booking.zerolatencyvr.com/book-now/sydney/
$60 pp for 30 minutes.
Go Karting
http://extremegokartingsydney.com.au/arrive-drive-pricing/
Out in Villawood. $35 for a 10 minute session.
iFly Indoor Skydiving
https://www.iflyworld.com.au/
About $390 for 5 people to do 2 'flights' each. An extra $15/pp if you want them to zoom you up and down in the wind tunnel.
A flight is 60 seconds. Sounds crazy short, but the freefall part of an actual sky dive lasts 30 to 60 seconds (depending on the height of the jump from 10,000 to 14,000 feet).
Quad biking
$120pp for 90 minutes.
https://glenworth.com.au/
Horse riding
$120pp for 2 hours. They do $120 for 4 hours school holiday kids deals.
https://glenworth.com.au/
Archery
https://www.archerycentre.com.au/Things-to-Do/Bronze-Arrow-Session
$25/pp for 1.5 hours at Sydney Olympic Park
Laser Tag
https://coderedlaser.com.au/
$20/pp for 2 hours on Sat/Sun morning or Fri/Sat night at Rosehill.
Bodyboarding and Snorkeling
$40 for a Bodyboard $20 for goggles and a snorkel.
Surfing
Escape The Room Physical
1 hour for about $45/pp. Normally a room with a bunch of cool puzzles. There's generally a story, but the story isn't well defined because there's only 1 hour and a load of puzzles to get through. We did Mission Identity at Gladesville. It was fun, but stressful because only 1 hour to get through it (they gave us an extra 20 mins or we wouldn't have made it), I can't really even tell you what the story was.
https://www.expeditionsydney.com.au/
Escape The Room VR
https://enterthemission.com/sydney/ $49/pp weekdays $59/pp weekends. Kent St, Sydney.
Bouldering
The good thing about bouldering is you don't need someone else on the line keeping you from falling. You might need to hire shoes, but don't need a harness.
Umbrella Nine is great for kids under a certain height - about 12 or ideally younger.
$21/pp Freshwater 5.0 Google Stars https://www.skywoodclimbing.com/pricing-memberships
$21/pp 9 Degrees 4.8 Google Stars https://www.9degrees.com.au/adult-pricing
Indoor Rock Climbing
You'll need pairs cause you need someone on the line in case you fall. Some indoor rock climbing places also have bouldering areas.
$26 inc. harness Brookvale 4.6 Google Stars https://www.rockhouse.com.au/
Tennis
Some racquets and balls. Easy. Don't hire a court, just go down and wait for a free court.
Kayaking & Stand Up Paddle Boarding
https://www.prokayaks.com.au/hire/ $55 to hire a double kayak for 2 hours.$40 for SUP for 2 hours.
Narrbeen Lagoon.
https://www.prokayaks.com.au/hire/ $70 to hire a double canoe for 2 hours. $50 for SUP for 2 hours. The Spit.
Luna Park
About $55 per child and $65 per adult for unlimited rides pass. https://www.lunaparksydney.com/
High Ropes
https://treetopsadventure.com.au/faq/ $51/adult $41/child. About 2.5 hrs. St Ives.
Movies
Bowling
https://www.strikebowling.com.au/locations/strike-macquarie $32/pp for 2 games
https://www.zonebowling.com/ $25/pp for 2 games. $33/pp for 2 games and 20 minutes on the arcade $10 cafe voucher.
Public Swimming Pool
https://www.ryde.nsw.gov.au/RALC/Facilities/Aquatic-Recreation Check the inflatables availability.
Raging Waters (Wet and Wild)
Paintball
$100 for 700 shots. You may need more. Whole day activity. Probably only good for boys. Milperra.
https://www.paintball.net.au/
Learn to sail
Manly $500 for a course pp for 8 weeks. https://www.manlysailing.com/kids-sailing-lessons/
You can hire a Laser Bahia Dinghy at The Spit. $175 for 2 hours. But you need a license to show you won't drown.
Shopping
Give them some money and go to the shopping centre.
Camping
Wake Boarding
Surfing Lessons
Trampolening
Moonlight Cinema