Sorry I haven't checked back here in a while I've been busy. I went with large river rocks which they seem to love. Now that they can get in and out they never do just chill in the water all day/night for the most part.
It looks like it takes up more room than it does, they still have tons of water area. The one in between the rocks goes in when startled but can get out easily. I made sure the shake the rocks so they settles so no real risk of them moving/getting trapped
I have a heater in there and it keeps it a steady 78-80F
DeeDub I love your setup, what size tank do you use? I plan on upgrading to a 75+ gallon tank for the 3 tank mates or maybe move to 300 gallon for a full viv/terrarium look with a nice big water area in the center like a pond or something. Then I can have all 5 together when adults and have a few more additions.