Food and Trader Joe's in Princeton [moved from Tristate board]
The Trader Joes in Philadelphia would be your best bet. The closest one to Princeton, in NJ, is in Marlton, not exactly on the way from the airport. The one on Phila is on 20th and Market, will take you on at least a 20 minute detour from the airport. I suggest you stick with the Whole Foods or the Wegmans in Princeton. Both are right on Route 1, the main highway going through the Princeton area.
I believe there is a Trader Joe's in South Jersey, but it might be more off the beaten path if you're taking 95 north up to Princeton. There is a Whole Foods (on route 1) and the Natural Health Food store on Nassau (on 27- it's renovating, so I don't know if it's open for renovations- I didn't see when I drove past it the other day).
As for reasonable dishes and casual atmosphere;
Masala Grill on Chambers Street: Indian food- a lot of people like Kalluri Korner more on Nassau, but I like the fact that Masala Grill is off on this side street. The Butter Chicken is awesome, as well as the Chicken Tikka Masala.
Triumph Grill and Brewery (on Nassau, next to Panera): I'm not a beer person, so I can't say if their beer is out of this world, but I do enjoy the Honey Wheat and Cherry Stout. The food is OK, I find it a bit overpriced for nothing too spectacular, but it is good nonetheless. Its an overall nice place to eat and drink with friends.
Theresa's Cafe Italiano- Palmer Sq, right across the street from The Palmer Inn-they offer artisanal pizza, about 10 dollars for a 10" (?) personal pie. I haven't tried any of their entrees, which are reasonable priced as well. The wine list is OK. They use a lot of organic and local produce and products.
Alchemist and Barrister- Witherspoon street- the bar menu might be more in your price range. I haven't eaten here, but the nighttime bar crowd attracts the princeton student crowd.
Sotto-reasonably priced Italian food- (downstairs on the corner of South Tulane and Nassau, I believe). Open late.
If you're looking for dessert or something sweet post-dinner, definitely stop by The Bent Spoon (Palmer Sq.) as they have the BEST vanilla ice cream ever. I think Bon Appetit or Gourmet Magazine bestowed them with this honor. They also have a bunch of great, creative flavors of ice cream and sorbet like heirloom tomato. The cupcakes are fun too. Halo Pub is also a local dairy farm, and is slightly cheaper.
I do not recommend the Underground Cafe (Hulfish St. - it's kind of pretty inside. handmade glass tables or something like that, but the food...meh. it's a bit overpriced), Ichiban (Hulfish St- there is better sushi on the go on Witherspoon). ), JJ Winberies (Palmer SQ- I am just so sick of the place, the food is allright, nothing too exciting).
Agree on Masala and Theresa's, though the latter does not take reservations so be prepared for a wait on Saturday night. Another possibility would be Soonja's on Alexander between University and Faculty (east of the Wawa).
If you really must get to Trader Joe's the one in Marlton is not far off 295 so you could take the Walt Whitman bridge after leaving the airport and come up the Jersey side of the river to Princeton.