ok the first plant that I think would be suitable and give you a third dimension to the bowl is Low Bulrush Scirpus cernuus (growth 8inches/ 20cm) which may still be too high. Also Fairy Lily Zephyranthus candida, but this will like it a bit dryer and it does tend to multiply bulbs like crazy, which is fine if you have a garden you can plant extra bulbs into.