Walls near to the cage should be washable as droppings and bits of soft food are sometimes flung out of the cage. I have constructed cages that have clear plexi-glass as the back wall of the cage. This helps prevent some of the mess. Choose a cage that is easy to wash, as it will need frequent cleaning. The cage should include a variety of perching materials.
Branches are preferable, as dowel sticks tend to be too smooth.
I would encourage the use of a feeding platform in both aviary and cage. Mousebirds can be messy eaters and do not hesitate to dig through the food bowl to find their favorite tidbits. Mousebirds like to moisten their food before eating so water should be nearby. Provide colorful toys for the pet or single mousebird. Here in Southern California mousebirds easily acclimatize to outdoor aviaries.