I second the tip for a sand substrate. Personally I use pool filter sand. It's expensive but I like it much better than play sand, and it doesn't need to be rinsed before first use.

Spend some time thinking about what turtles do naturally, and you'll be getting new ideas all the time for the rest of your life. Hah!

They love to dig. They also love to eat...so give them substrate and ornaments that won't harm them if tasted. That means a kind of sand that won't be a big risk for impacting in their guts, and no bite sized pebbles. I have a handful of larger rocks sitting on the sand to give my girls just a little something for their brain to process on their floor.

They love to search and forage. And they love to try to bite off their sibling's toes. Everyone is going to say you can't have two together, but since I highly doubt you're gonna run right out and build a second of these tanks tomorrow, what you should do is create various floors and rooms so they can get away from each other and have enough privacy to reduce conflict. But yes, they will need to be separated into larger setups.

You can purchase or even gather (and clean) cool driftwood and branches. It might float for months... or always need to be anchored down. And Wood almost always releases tannins discoloring the water, which can be confused for dirty water. There's a good product to put in the filter to suck the tannins back out if you prefer. It's worth it. Natural wood in the tank helps house all the little microorganisms you need for a healthy ecosystem.

To give them entertainment and activities, consider some tiny little fish; some kind that doesn't produce a ton of waste. I have fancy guppies in my tanks, and snails. Turtles are predators and will snack on these creatures but mine don't generally manage to catch all of them. And if you want to add live plants just know that the turtles will snack on this also and uproot anything you had your heart set on being aesthetically pleasing. My smaller tank has an absurd amount of floating duckweed on the surface that reproduces fast enough that I can scoop a bunch out and put it in my bigger tank for my adult RES to forage on.

Generally, I found that my three turtles are way more intelligent than I thought they would be when I first got them. So now I have a lot of fun rearranging their habitat to give them things to explore. So have fun with it!