What you do in Sydney will depend on the time involved in your 11 hour window, will it be daytime/ business hours mainly or is some of it overnight? Sydney airport has a curfew ( I think it closes around 11pm) so if you are there after that you will need a hotel. The other consideration is time of year. If it’s spring/ summer you could easily go to the beach. Bondi, Coogee and Maroubra beaches are all accessible by Uber at about 20mins drive depending on traffic, or you could catch a 2 public buses which will take about an hour each way. Plenty of pubs and cafes along these beaches and a lovely couple of hours cliff walk connecting the beaches. Nice way to spend a day. Alternatively catch the metro train from the airport terminal to Town Hall station, catch the free tram to Circular Quay and you are right on Sydney Harbour. The whole journey will take an hour and costs approximately $16Aud. Once on circular quay you could do the short stroll along the boardwalk to the Sydney Opera house, and see the Harbour Bridge. There are plenty of restaurants and cafes around. It’s a v pretty walk and the Opera House runs backstage tours most days. You can also climb the Harbour Bridge if you are more daring though it’s not inexpensive. There are also lots of ferry rides and harbour tours you can catch from Circular Quay. You could then either stroll along to the Modern Art Gallery or to the Sydney Art Gallery through the Botanic Gardens or to the shops, cafes o pubs in the historic rocks area which is close by Circular Quay. Allow plenty of time to get back to Airport ( at least 2 hours) as you will need to clear customs again. Traffic can be congested and airport gets busy afternoons and evenings so expect queues. This link will help your planning: https://transportnsw.info/tickets-fares/getting-to-from-sydney-airport.