Slimfast fruit & cereal bars are under 100 cals and Asda have them on offer at the moment.
If you are planning on having a daily sweet treat, don't be tempted to just buy a packet of biscuits with the intention of only have one or two each evening ... we all know where that will lead!
Get things that are individually wrapped so you can keep a conscious account of each one you have.
Another suggestion would be to make low sugar jelly and pour it into individual bowls so you can keep tabs on the portion sizes. A little bowl of jelly with a dessertspoon of low fat ice-cream might hit the spot.