Have you done the hiding toys, balls etc already? You can make that a bit more challenging by hiding the object in a shoe or basket or under a cushion etc.
If you've got any old plastic beakers, you can hide treats under them and let him sniff them out, its pretty easy for them to tip the beaker and get the reward on a carpet but on a tiled floor its a bit harder and needs working out!
You can vary it by not putting treats under all the beakers but just some. You can also line them up, just put a treat under one and then point to it and see if he can make the connection between your indication and tne treat.
A treat wrapped in screwed up paper will keep them busy for a bit, - you can hide several in more paper and put it all into a big cardboard box.
You can also do some indoor exercises, training him to sit on your left, your right or behind by following the treat in your hand. See if you can find some tricks using clickers on the internet - they're usually pretty non energetic things, some useful some are just party tricks but they'll help keep him stimulated.
Hope some of those might help......!! Get well soon Rhodri!