Depends on transport availability. As Clarence says there's a lot you could do in Morton NP - Yalwal to Tallong springs to mind but that's a pretty lengthy car shuffle. Could also do a full circuit of the Buds, eg. linking Hollands Gorge with the tops over Mt Tarn etc - heaps of variations/options, but (also as Clarence says) depends on your appetite for off-track.
Regarding the question about linking the 6FT with K2K.... no need for a road bash if you jettison the leg of the 6FT between Jenolan Caves and the junction of Black Range track with Moorara Boss fire trail, and instead turn down the MB trail and link with the fire trails between Kanangra and Moorara ridge, across Thurat/Whalania heights etc. There is a brilliant campsite on the banks of Jenolan/Harrys river where the Moorara Boss trail fords it (I got snowed on there once
). Can make it a circuit ex-Katoomba then, or alternatively ex-Kanangra if you either find a way to link the Megalong Valley with Konangaroo (eg. down the Cox or using Blue Dog to Mobbs then down Yellow Dog... plenty of variations), or just use Katoomba as a half-way pit stop and go via Narrow Neck and Nellies Glen. Another option is to avoid the 6FT altogether and make a circuit with the K2K using the Krungle Bungles or Jenolan River. I've done some but not all of these routes so others might have more/better advice.
Can't comment of 3 peaks but it's on my list as a slow walk as well