The PC's, after wandering into the nearest town for some R&R, suddenly find themselves drafted into an expeditionary army as a scouting party or even a small, expendable unit with an NPC leader. The pay is a little money plus food and any necessary clothing. If you want to be nice, you may assign the PC's horses, if they don't have any. As a scouting party, the PC's don't have to travel with the main force of the army, which gets rid of the possibility of huge battles.
At this point the PC's have several options:
Any way they choose, you can follow up with new ideas or just adjust the outcome so they wind up back in the army. The overall goal of the army is up to you. Whether it is to rescue a princess, lend aid to a besieged town or outpost, explore uncharted territory, or even to defeat an opposing army, the PC's need not participate in any large-scale battles. The job of scouting gives many opportunities for encounters. Wilderness encounters, encounters with enemy forces, a ruined temple, or a castle or two, are just some of the things that can be encountered. Nature itself can provide lots of good role-playing opportunities. For example, do you make the dangerous trek over the mountains or go around? How are you going to cross that rain-swollen river? The possibilities are endless.
The total outcome of the whole campaign can also be the basis for another adventure. What happens if the army is defeated or routed? Do the PC's try to carry on and compete the mission? What will the PC's do when they find themselves stranded deep in hostile territory or deep in an uncharted wilderness? If the campaign is successful, will the PC's be tempted to split up by being promoted to higher positions in the military? Will the PC's distinguish themselves and become heroes or celebrities? Will they fail and be looked on as traitors and criminals? The rewards can be great and so can the risks.