The characters will find themselves tied up to trees. What skill check will they need to use - Athletics?
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Escaping is an Acrobatics check.
Set the DC according to the difficulty of escaping from the bonds (set the difficulty as easy/hard/medium and give it a level, then go the the DC table and see what it should be).
Depending on how you've tied up your PCs, athletics may also be appropriate (see Escape section linked and quoted below). Generally though Acrobatics is preferred here.
If you would like you can set one skill as the easy or medium (wriggling from the bonds is easier than breaking them or some such) and one as medium or hard to give some consequence as to how your PCs choose to escape.
Something else to consider, if this is an out of combat scenario, you almost certainly want to work this out as a skill challenge. Design it so that athletics, acrobatics, and perhaps thievery are primary stats, and things like perception, endurance and another skill are secondary things.