1.) You probably have some BBEG to thwart or some threatened goody institution to save in your campaign notes that could be tied into this mission.
If you don't have, I would seek inspiration from the old German urban fantasy RPG angeli: honorabilis lex ludi where angels and their allies seek to advance God's Plan(TM) (and The Adversary and his hellish minions seek to thwart them). Let the PCs undertake a seemingly inane mission, such as saving a blind child from being run down by a carriage by day X, with that child playing a great role in the angelic plans in the future and making discovery Y in the future that will forward the schemes of the lawful good deities. Or let them make sure that the blind child gets overrun by a carriage on day X, to prevent it from growing up and giving decisive aid to the forces of the lower planes on day Y as pre-determined by its fate.
Rather than traditional loot, players could acquire angelic boons, such as access to a set number of divine phrases (that effectively emulate scrolls) such as "Amen" (a beefed up Suggestion to simply accept what has just happened and not to look any further into it), "Look up and see" (Remove Blindness/Deafness), "Stand up, pick up your mat, and walk" (Remove Paralysis)...
2.) Planes of opposite alignment are very painful for invaders and come with severe penalties on all rolls. 2E Planescape had some even harsher setting rules and especially punished clerics by reducing their effective cleric level the further they strayed from the home plane of their deity.
Actually 2nd's Spelljammer ships can enter the flow using Phlogiston which allowed them to fully cross entire campaign settings, like moving from Dragonlance to Faerun.
I was thinking of 2E here, when the three featured campaign settings Faerun, Krynn and Oerth were planets within the Material Plane, and the Material Plane also consisted of the space, the phlogiston and the celestial bodies between these planets.