Tricky as you only have half a day or so.
Much as I like Wentworth Falls (hey I live there!) I would give it a miss at the moment as the main car park near the falls is closed for renovation
ATM. That means you have at least a half hour walk each way to get there. Normally I'd recommend the Darwin walk but it would eat up a good part of your time budget.
Three Sisters is top notch but VERY touristy. The only pay parking near any of the lookouts.
THere is also tourist alley where the Scenic Railway, cable car etc give you easy access to the valley floor WOrth a visit ONCE. THe scenic railway is a do once.
I agree with ATJ about the L/Os near Blackheath, though they don't overlook the Grand Canyon, they look over the Grose Valley. The GC is a much smaller offshoot of the Grose, & best experienced by splashing along the bottom, but this is a day trip on its own, and best experienced in summer.
If you do go to Blackheath, a little known spot is Mermaid's Cave. This is on the road down to Megalong Valley. on the West side of the Railway.
It is about a 10minute walk from the road (calling the parking a car park would be generous in the extreme) and maybe 15 Min back up.
Another spot with great views is Sublime point at the back of Leura.
Perhaps the best option would be to drive as far as Lithgow on the Great Western Highway, then drive through Lithgow to the Bells Line of Road, and follow it back to Windsor via Bilpin and Kurrajong. You will want to take small detours to see the views. This will take you past Lawson, WF (wave!), Leura, Katoomba, Medlow Bath, Blackheath, Mt Victoria, Hartley Vale, Zig Zag railway (different to the Scenic Railway), Mt Tomah (botanic gardens also detour to Mt Wilson), Bilpin, Mt Kurrajong (wind down the windows & listen to the bell birds), Clarendon (The RAAF airbase can be a photographer's delight at times. other times it is drab & boring), Windsor. This will take a long day though, or several if you stop a LOT.
The best Fish & Chips are at WF opposite the station, the hamburgers are good too; though the most spectacular eatery by far would have to be the Hydro Majestic at Medlow Bath. You pay handsomely for the ambience though.
Be aware that the traffic can be horrendous at times especially Sunday afternoons travelling east on the Great Western. My kids have given up visiting for lunch on weekends as it takes hours to get home. I have made the mistake of going up to Katoomba then. 10 minutes to get there; an hour and a half to get home. And I know the back ways.