It’s a good idea to prepare a map of sorts. It doesn’t have to be super detailed. It’s mostly to figure out what type of terrain they are going to run into when they do get outside the city and that will be a big help to you. Having a list of names for towns, villages, forests, lakes, etc would be a big help as well. You can also use something similar to my random event roller to come up with quests outside of the city. Try and give the players a big goal that may last a few games to complete as well. Try and make this big quest change the world they are playing in, in some way. Maybe it involves a change in rulers or maybe it involve an important place being destroyed. It’s important to make the players feel like they are important. another thing that may be a good idea to make the terrain less boring would be to make a list of sights and little oddities like ruins, caves, and other things like that. This is where you can use a random dice roller. As an example, let’s say the players leave the city and pick a direction to go in. obviously, they may not run into anything immediately but after a few minutes of exploring and describing the terrain, you can make a roll. I would set it up like this and use a D20
1 nothing happens
2 monster encounter
3 NPC encounter
4 Nothing happens
5 Finds something ancient (ruins, item, etc)
6 Finds a town, village, or city
7 Nothing happens
8 finds a camp
9 gets caught in a trap
10 finds a note, map, weapon, or other object
11 something magical or paranormal happens ( a village shrinks, statues come alive, etc)
12 Nothing happens
13 monster encounter
14 finds a mystical beast
15 finds a cave or an entrance into an underground area
16 finds a staircase
17 nothing happens
18 monster encounter
19 finds a bunch of giant (spiderwebs, claw marks, etc)
20 runs into a political figure or something that has to do with politics
Obviously, you would have to come up with the details yourself but hopefully this helps you get started.