Keep some quiz books in you bag. I have some that are differentiated according to age so a suitable quiz is always ready. For a story keep a book or two in you bag. Try to find some that are not commonly used to avoid the chorus of 'We've done that one already'. For older classes consider some historical stories. A great series are available in Tesco and Easons (as well as other bookshops) of different Irish myths, legends and stories of Irish heroes including some of the favourites - Children of Lir, Salmon of Knowledge, St Patrick and Brian Ború. I love these books and have used them with all class groups from infants up. They can also make a fairly simple History lesson. Keep a book of brainteasers and puzzles with you as well. They can be photocopied or projected onto the board if suitable. A discovered on my last TP that 1st class loved a game of bingo if there was time in the evening. Get them to pick 9 numbers and using a 100 square (I love this one http://www.primarygames.co.uk/pg2/splat/splatsq100.html as you can keep track of what numbers have been used and it can be done randomly, also it can be projected to the board so the children can keep track of it as well).
It's lunch time on your day subbing and you look out the window to see it is raining. The children normally watch a film and there is no access to youtube - a nightmare. Keep a DVD with you and this will be avoided and you can head to the staff room to make the most of the break.
The final thing to bring with you is the most important - your lunch. Don't forget a bottle of water to avoid the dreaded teacher's throat.
Now that I have planned what I need to bring I just need to get it organised :)