Lost in the woods, need rope... Find some nettles, boil them, eat them, and take the fibrous "strings" out. DC:15 Rope Use to weave the fibers into rope.
Cut some saplings for poles, lean two long ones towards each other (so it's wider at the top than the bottom), tie some crossbars to both sides, and you have a ladder...
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(The slant is to keep the bars from pulling downwards, when you put your weight on them).
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Building boxy structures... Huts, four-sided towers, etc.|_||
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Building frames. A litter/travois to carry someone on, etc. |X|
Taking a bunch of de-barked logs, and building a stockde.
ooo
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Setting up a Deadfall (DC:15 Survival), where something falls over on top of'em; Simple Snare (DC:2), "Jerk'em Off Their Feet By Whatever's Caught" Snare (DC:15), A "Sharpened Limb That Hits'em Like A Morningstar" Trap (DC:15), or a "Drop The Lid When They're In It" Pit Trap (DC:15). You can also combine a "Jerker" and some sharpened limbs to jerk them INTO the spikes... "SNAP! Whooooooosh-SPLAT!" Then there's the ever-popular "Spiked Deadfall", where the big, spiky log, with sharpened limbs, comes swooshing down outta the branches, and impales anyone along its path... Obviously, it is hanging from a rope!
Need to climb, but the odds are low, and no one has gear? How far you can throw a grapnel is based upon Rope Use, not BAB, so your PC will pro'ly be the best... Climbing a knotted rope is easier than a regular one. Also, he should be able to rig up a climbing harness for the +2 Circumstance Bonus out of a 10' piece. Also, if climbing a pillar or tree, you can put a piece around the other side, lean back, hang onto it, position your feet, then jerk the rope up, move feet, jerk rope, move, jerk... and thus climb the pillar/tree with a +2 Synergy Bonus for 5+ ranks of Rope Use...
Note: Most of this (except for the traps stuff) is in the rules... The traps stuff is just my take on it, based (in part) on the
Detect Snares and Pits spell description. Unless you need to throw a grapnel a looooooong ways, don't put more than 13 ranks in Rope Use... With DC:15, you can tie any knot, and you'll get at least a 1 on the roll, plus your stat add. Check with your GM, and his/her interpretation is what counts.
You will want high Climb, Survival, and Rope Use, as well as (depending upon GM) some Craft (Trapmaking). (Based on their interpretation, you may not be ABLE to make traps... The DMG says one WEEK and 1,000 GP per CR of the trap! This is a personal hangup, of mine!)

Some GMs may also tell you that you need Craft (Carpentry or Woodworking), for example. You'll just have to check with them, talk to them, and work out how you'll do it.
Building a suspension bridge isn't beyond possibility... although you'll need to add a good timber saw to your equipment list (to make boards). I suppose you could do it with logs, too, but it's trickier to walk on, that-a-way!
There was a recent thread, just a few days ago, about building a raft... Somebody with Search want to find a link to that? ===>
Survival shelters, and just plain rigging a tent, can be done with Survival or Rope Use, either one.
I think 5+ ranks of Rope Use gives a +2 Synergy Bonus to Escape Artist (to help offset the -10 penalty for being tied)!
Some Indian tribe also had a whole system of telling a story through knots... Sending messages that way might be cool, but... who would read it?
Honestly, I think hitting something with a Lasso or grapnel should be handled by BAB, not Rope Use, and that it (Rope Use) should be removed from the game. It is never worth the investment in skill points, IMHO. PCs should just be assumed to be capable of tying the proper knots!
