Grab an apple or 2 at breakfast and snack on them during the day. If you skip meals you tend to overeat later so at least 5 small meals a day is what you need to shoot for.
Cheese with fruit is a good protein/energy hit.
You can buy nuts in 6 packs and have a couple of packets in your briefcase.

Sandwiches.... they're what you make them. A wholemeal salad sandwich with cheese isn't a bad lunch choice.