Cheap eats and Solo eats in Center City/Rittenhouse in Philadelphia?
Welcome to Philly. As a new-ish transplant leaving for Boston next week, I am really jealous of all the fun you're going to have! I recommend Almaz Cafe, an ethiopian cafe at 20th and Walnut or Giwa, fresh Korean food in a bright, clean, casual space at 16th and Sanson. During the day, the falafel stand at Market and 20th with the wood-burning grill - best falafel I've ever had. Happy eating!
Second on Almaz Cafe--very good Ethiopian food. It wasn't on the menu when I visited, but if you want to have a sampler dish, just ask and they'll be happy to accommodate you.
The bar at Tria at 18th and Sansom is perfect for solo dining. They have wonderful and interesting cheeses, panini, beers and wines that won't break the bank.
Black Sheep, at 17th and Latimer, is a cozy pub with excellent burgers and mac 'n' cheese.
Tampopo is great for $5-7 range, rare in Rittenhouse, and they have Japanese and Korean options. For better Korean food at slightly higher prices ($8-11), try Miran close by on Chestnut between 20th and 21st. On a lazy Saturday morning, get a croissant and coffee from Le Bus (on 18th just above Walnut) and eat it in the park.
Chinatown is relatively close (15 minute walk) and has a high density of cheap eats. For solo dining, try Nan Zhou's hand drawn noodles (Race between 9th and 10th) or assorted pastries from bakeries like Mayflower (Race between 10th and 11th).
Also check out this recent thread on breakfast sandwiches in Rittenhouse: