Slippery Elm (bark) is a wonderful, mild all-round remedy. It's great for constipation and diarrhea, for respiratory conditions and indigestion, etc. (See below for more info) It comes in a powder form, has a pleasant smell (like icecream cones!) and tastes slightly sweet. Be sure to buy organic.
If you want to turn a fruit smoothie into a firm dessert, add a little Slippery Elm to the mixture in the blender.
How much? Allow 1/4 or 1/2 tsp per person. Pour the smoothie into the containers and give it a few minutes to set. Re-blend with a little more Slippery Elm if the dessert doesn't set solid enough. You may find it continues to set, the longer you leave it - let me know! I usually eat my smoothies straightaway.
To support your digestion, eat fruit desserts before you eat the rest of your meal - a great way to begin the meal anyway: you get to eat the best part when you're hungry!
Some helpful info on Slippery Elm: http://www.drksmith.com/slipperyelm.htm