The care article: Frog Forum - White's Tree Frog Care - Litoria caerulea suggests a tank with water as a substrate with climbing areas above the water.

I've found several wild Hyla versicolor that seemed to be stuck inside plastic rain barrels. Despite the trouble they seemed to have climbing the plastic when wet, we never found a drowning victim (different frogs, I know!). We now leave sticks in them for an easy way out and haven't found a trapped one since. Easy out of a water feature is always good!