Cradle Mountain-Lake St. Clair is pretty easy. A ton of boardwalk nowadays.
If you get the chance, the South Coast Track is a much better challenge. We flew into Malaleuca airstrip, then drove out from Cockle Creek. We saw only a handful of people during the 4 nights we took. I think the average is 5000 per year for Cradle Mountain, and 2000 per year for South Coast Track. Ironbound Range is the most physical part (coming back down the other side on very tall "steps" is a bitch on the knees). Much more primitive than Cradle Mountain, no fancy huts, only boardwalk is closer to Cockle Creek. You can really get your feet in the mud with this walk. Rowing across New River Lagoon is icing on the cake (you need to go over twice, once to get the other boat, bring it back to where you first got on, then back across again).
Highly recommended if you are capable. This was done before they introduced the permit system, dunno if you need one for South Coast.