Good idea of easy, healthy, one-handed snacks. I was forever starving and knowing I was sitting down for a session would often grab biccies - and wondered why the weight wasn't dropping off even though I was feeding a huge boy! So I second bags of nuts/seeds/fruit, healthy snack/cereal bars, bottles of smoothies/juice (to prevent constipation).
For entertainment, some good books or mags, DVD's (series are good; I've already got my Mom's ER box set lined up for October, or if she likes sex and the city etc).
The insulated mug is a great idea crikeybadger - you could get her this or a fancy cup/lid/straw combo (think Starbucks have one for sale), which she could keep filled up in the frisge ready to grab when sitting down for a feed. Means she gets enough to drink and less likely to spill!
A breastfeeding cushion if she hasn't already got one
A small nightlight for night feeds. I used ot scoop my sons into the bed and feed next to hubby, and in the first few weeks you need to be able to see to help them latch on, so I got a small glow light which was enough to see by, but not enough to be harsh for me or wake hubby. If you're feeling really generous a radio/ipod with headphones in case she's wide awake doing night feeds!