Simple Variants
From World Of Catan
Desert Resources
A way of making the game easier is to abolish deserts, instead making them a resource hex -- either clay, or ore, or a special resource called "mineral sand", which can behave either like clay or ore.
You need to take a spare blank number token and write a number on it (possibly something like 4 or 5) and add it to the pool of number tokens and distribute it along with the others. This is easier to do if you are using the "semi-random" method of token allocation (pull the tokens from the pool at random, and if you get either the same numbers next to each other, or a 6 or 8 next to each other, put the token back and pull another one).
To use "mineral sand" you'll need to make yourself a set of "mineral sand" resource cards. A mineral sand card can be used as either a clay or an ore. Which actually makes it the most useful card on the board, so maybe this variation is a bit unbalanced.
Absolute Longest Road
A variation used by my other brother, which makes the "longest road" harder to keep. If a player has the longest road, instead of keeping it until someone else makes a longer road, if another player makes a road of equal length, then the "longest road" card goes back in the box, since nobody has the absolutely longest road.
Presumably one could do a similar thing with "largest army".