This is a perfect combo of my two favorite ideas for that space under the stairs leading to the basement. A reading nook AND book storage and shelves . assuming that there is enough space for this there.
natural small garden ponds pond is one of great small garden ponds ideas. You can make it by using PVC lining and place it with some rocks around it and decorate with water plants. In order to beautify it, you can raise some fish in it, such as