Unfortunately there isn’t much. After the previously announced deadline of March 31st the only thing left on the DSi Shop is the 3DS Transfer Tool and your purchase history. If you would like to keep your DSiWare games accessible re-download them or transfer them to a 3DS as soon as possible. This ability will also be retired at some point but Nintendo has not yet given a specific date.
You can look back at the DSi Shop in action with my previous post and this YouTube playlist recorded before the deadline. This gives us a historical view of what was available directly on the DSi up to its end and can be compared to the current offerings on the 3DS eShop.