Games Tweaks
Here's a list of alternative rules you might like to try.
Don't allow the robber (on a throw of 7) for the first three turns (re-roll 7s).
Settlers of Catan
 Take 11 of each commodity and shuffle together to make a draw deck. Take one (or two) cards at the start of your turn.
Re-number the "2" and "12" spaces to be "5" and "9". If you throw a 2 or 12, draw a commodity card from the draw deck and roll again.
Re-number the "2" and "12" spaces to be "5" and "9". If you throw a 2 or 12, draw a commodity card from the draw deck and roll again.
Roll for mine production twice each turn to even out the dice roll odds.
Boomtown
Discard any corn in the initial plantations.
Change the University so it costs 6 and scores 2.
Puerto Rico
From the expansion set, add in Storehouse, Lighthouse, Library & Guesthouse. Remove Office, Large Warehouse. The Guesthouse is particularly useful, as we've found that anyone buying it invariably loses the game.
Let Jackie win.
Use the hand cards to allow players to cancel stacks of cards. Also, hand cards do not count against your score.

Do not allow players to discard cards to the centre - a card MUST be played every turn!
Lost Cities