You'd probably get some more directed help if you gave us some idea of your fitness and experience, and also where you want to camp (near water, high camp with views etc). In general, campfires are allowed in the Greater Blue Mountains area. However, in some heavily used areas (eg Acacia Flat in the Grose Valley), campfires are not permitted.
Some possibilities:
1. Glenbrook to Springwood - camp near Lost World Lookout, need to carry water
http://bushwalkingnsw.com/walk.php?nid=7282. Splendour Rock - again, need to carry water
http://bushwalkingnsw.com/walk.php?nid=8043. Knights Deck - camp at junction of Breakfast Creek/Coxs River
http://bushwalkingnsw.com/walk.php?nid=812None of these are particularly hard; many parties do them as longer day walks. Tracks are not marked, so you need to take a map and compass and know how to use them.