Hi Beth,
Hope you're enjoying your visit to Sydney!
The best maps to use for bushwalking are the NSW LPI 1:25000 scale maps. They're green and white, and have the best compromise of coverage, detail and scale. You can look at the maps online here:
http://maps.six.nsw.gov.auClick 'Basemaps' in the top right corner, then 'Looking for 1943 Imagery?', then 'Topo Maps (current)' in the resultant menus.
If you just want an overview of the Blue Mountains, the Spatial Vision Blue Mountains North and Blue Mountains South maps are pretty good. Nice maps with a good overview, but at a 1:50000 scale.
There's certainly plenty of great overnight walks around Sydney, especially in the Blue Mountains that you were looking at. However, to give you a better suggestion, it'd be good to know if you're restricted to public transport.
Two popular 'classic' overnight walks in the Blue Mountains are Mount Solitary from Katoomba, and Blue Gum Forest in the Grose Valley (there's many ways down to here).
There's some other good options around too, but both of the above walks are very scenic, and a good spot to start in the Blue Mountains
In the Royal National Park, the Coast Track is quite popular and scenic, but you have to book it, and it is frequently booked out well in advance (especially on weekends) as there is only one campsite. There's other overnight tracks in the area, but if you've only got time for one, the Coast Track is probably the best option in the RNP and Heathcote NP.