Fruit and vegetables are great snacks. Fruit needs no special preparation other than washing. Vegetables can be washed and cut at night, but if you don't mind spending a little bit more money you can get packages of precut veggies and just wash them.
The great thing about fruits and veggies is that they fill you up quickly, when eaten alone as snacks they are best digested/absorbed, and they can help you lose the weight since they are fibrous and low calorie. On the plus side since you are nursing, many fruits have a high water content and can help with hydration (as you probably know it isn't the 'water' that increases your supply, but hydration helps with body processes and functioning as not to inhibit lactation) Also, fruits and veggies can easily be eaten in the car if you're on the go. To mix it up and add a bit of protein, make small baggies of nuts like peanuts, almonds, or cashews.
If you absolutely must have sweets, try the various 100 calorie snack packs out there. Granted, they're not great nutritionally, but they can satisfy a sweet tooth without blowing your calorie allotment.
When running errands, it is still okay to go to places like Dunkin Donuts, McDonalds, or some other place with a drive-through, but know that many of those places are starting to get with the program and offer healthier snacks (such as the fruit and yogurt parfait from McDonalds). Nearly every fast food restaurant lists their nutritional information online, so check out your favorite places and see the 'good stuff' they have to offer and order that instead.