for some quick bites to go, i recommend two of my fave places that were introduced by my local friend!
MANGO TANGO - for all things, umm, mango, and they serve up one of the best mango sticky rice (Khao Niaow Ma Muang) too! (Siam Square soi 5, near Hotel Novotel Siam Square)
MONT NOM SOD - fresh milk and thick toast with various spreads, like condensed milk, chocolate and marmalade. great for an afternoon snack! (MBK, Level 2, towards the exit to Pathumwan Princess Hotel)