In Northampton I really like Zen, a new pan-Asian place on Main Street, just after the Bridge Street underpass. The food is fresh, light, and well presented.
If you want a creative sandwich on artisanal bread, with excellent coffee as a bonus, try Woodstar Cafe, at 60 Masonic Street (if you're coming from I-91, head into town on Main Street, then make a right on Masonic. There is usually some on-street parking in the vicinity.
In Holyoke I can't make any specific recommendations, though I do know it's a good place to find both Polish and Puerto Rican mom-and-pop places.