How can I plan to spend my one day work-Break Sydney (Australia) ?

I have many ideas and I suspect others can also advise. But, what do you like?????? Outdoors, indoor museums, animals, shopping, architecture other???????? If you specify, probably people can help better.

do you mean summer in Australia or summer in the northern hemisphere?

It would help to have more context of your personal interests, as well as "summer"- as per Nigel. About 5 years ago I went in late July (from US) and while Sydney wasn't as cold as Melbourne it's still cold.
Anyway, two suggestions- the first is that I extensively used Trip Advisor with a forum specific to Sydney and at the time there were several reliable contributors I found very helpful.
My own interest is Aboriginal history and especially the Dreamtime. I started Day 1 with a walk to meet:
https://dreamtimesouthernx.com.au/Who-We-Are/Margret-Campbell
and saw the Opera House along the way. Her tour was powerful and provided a perspective I never would have experienced otherwise.
Then I walked around the outdoor First Fleet area.
For the rest I think it would be helpful to specify your interests. The shoreline walk and seeing Bondi Beach was memorable, too.

You might consider doing one of the HoHo bus tours to get an overview of the neighborhoods, attractions, and highlights of the city... riding thru one full cycle (usually a couple of hours) then hopping off at whatever interested you the first time around. My only caution is that the bus ride can turn into an expensive nap if you're jet lagged and sleep deprived to begin with.
Alternatively, or maybe in conjunction with the HoHo bus, you could purchase an Opal Card and ride the ferries across the harbor to some of the outlying suburbs. Manly and Bondi are two that immediately spring to mind, but a quick google search will turn up a host of other possibilities.
We particularly enjoy taking the ferry across the harbor to the Taronga Zoo ferry stop, then walking along the pathway that forms part of Sydney Harbor National Park - essentially a greenbelt that circles the northern part of the harbor and which features picture postcard views of the Harbor Bridge and the Opera House. Depending on how ambitious a walker you are you can hoof it for a mile or two (or more), enjoying the views and then catch a return ferry back to Circular Quay from one of the other outlying ferry stops that are dotted along the north shore. Makes for a very pleasant day out.
A stroll around The Rocks area and Circular Quay to the Opera House and the Botanical Garden is always nice - better if there isn't a cruise ship docked at the Quay.

Thanks for responding @ORDtraveler, I had planned a dinner cruise on Sydney Harbour https://www.australiancruisegroup.com.au/sydney/dinner-cruises. That's as far as my current plans go. Now for my interests, I'd say anything that's outdoorsy and let me stretch out my legs and open my eyes to new things. And since I'm doing the cruise I would prefer something not so touristy as well.

Hii, @NigelThe Australian summer I meant. I think from December to early March.

An interesting adventure is to walk across the top of “The Hangers”. Bridge. I’m terrified of open heights, so I didn’t personally do it, but I watched groups of people doing it.

I bought a ticket for the Opera House - a cheap seat right up front with a half-obstructed view, but a great first opera location!

I also took a ferry out to one of the beaches that also gave me a nice view to take photos from the water of the opera house.

I also just had a very quick weekend during a work trip. I stayed at a chain hotel at The Rocks during the weekend and liked that location.

I did the Bondi to Coogee Coast Walk in a morning. Popular walk, but lots of locals, not all tourists. Here’s one website. There is lots of info online. https://www.sydney.com/things-to-do/nature-and-parks/walks/bondi-to-coogee-coastal-walk

I did this route from Coogee accessed by bus from central Sydney. I swam at Iceberg pool before catching a bus return from Bondi.

Blue Mountains might be difficult to get back in time for your cruise, but you can check train schedules. Lots of hiking and outdoor sightseeing. Tourists, but not so many on trails. Locals again.

The suggest for the zoo is excellent. Great collection of Ausie animals, I have hiked in that area also if you don’t want to join tourist for the animals. Or, just take the ferry over snd back - worth it for views! The Botanic Garden is large. You can find a map online.

Good suggestions above for walking in central Sydney - The Rocks area and Circular Quay to the Opera House and the Botanical Garden.