Here are a few I've experienced in the past:

#Spiderweb Setup
Crisscross string or thin rope between two trees/poles to create a "spiderweb" pattern with various hole of different sizes, some near the ground and some higher up.

The challenge
The entire team must pass through the spiderweb without touching it, but each hole may only be traversed by a single person.

Generally there will be a few large holes that are easy to get through, but if they don't plan ahead the last person/people will be stuck trying to get through a small hole near the top.

If anyone touches the rope at any point, everyone must return to the beginning and start over.

Ways to modify for difficulty/number of people

• Allow certain holes to be used more than once, or allow a certain number of "repeats" (not assigned to a specific hole)
• Give them a pair of scissors that they can use a set number of times to cut individual strands
• Have some bits of rope that are easily removed so that you can make more/bigger holes if they're struggling too much

#Poison Yogurt Setup
Hang a small pail of water/sand/whatever with a handle on it from a tree or pole, at a little above head height (about 2m (6.5')). Draw a circle around the tree with a radius of about two metres (6.5 feet). Provide a stick about 1.5 metres (5 feet) long.

The challenge
The team has all been fed poison yogurt, and the only antidote is hanging on the tree. They must retrieve the antidote, but inside the circle is boiling lava, which will vapourize them instantly if they touch it (everyone must start over).

Ways to modify for difficulty/number of people

• Make the circle bigger/smaller (note that if the circle gets too big, the task will become virtually impossible)
• Make the stick longer/shorter
• Make a "rock outcrop" in the lava, which is a small mat/board just big enough for a single person to stand on. Don't make it too close to the antidote, or it will be too easy to retrieve.

#Troll Bridge Setup
Draw a starting line and an ending line, about 4-5 metres (13-16 feet) apart, parallel to each other.

The challenge
The space between the two lines is a troll bridge. The group needs to cross the bridge, but the troll has declared that no more than $x$ individual body parts (hand, foot, knee, elbow, etc.) may touch the bridge. Any more than that, and he will eat everyone (they will have to start over). $x$ should be less than the number of people (otherwise they can all just hop across on one foot). A good rule of thumb is to allow about $\frac34$ as many limbs as there are people.

Ways to modify for difficulty/number of people

• Modify the number of body parts allowed. Note that, especially for smaller groups, a change of even 1 can make a significant difference.
• Change the distance between the ends of the bridge.
• Specify which body parts are allowed (e.g. 3 feet, one hand, and one knee)

#Ice Floe Setup
Find a stump, crate, or sturdy box that can support the weight of several people and is not too large.

The challenge
The team has been stranded on an ice floe that is rapidly shrinking. They must support their entire team on top of the stump without anyone touching the ground, for 10 seconds.

Ways to modify for difficulty/number of people

• Use a larger/smaller stump/crate/etc.
• Change the time period they must remain aloft. Often a team will find an unstable configuration that they can hold for 3 or 4 seconds before they collapse.
• If there are too many people, allow a single foot (or hand or finger, etc.) to be on the ground.