To add some loops or some water splashes to your coaster, select a node in the coaster creator mode, tap on "Special insert" and choose the one you want to add. 

The amount you can place on a single coaster is limited. Upgrade that special insert card to place more. 

Be careful, special inserts add Excitement, but also add Intensity and Nausea to your coaster. Don't forget that a coaster with an Intensity that's too high will not have many riders.  

Note that any special insert will increase the coaster length, so you need to keep an eye to the limit that you have.