Grass jelly is a dessert of Chinese origin that can be found virtually everywhere in Southeast Asia. If you're not familiar with it, have a look at this piece I wrote last year for a domestic paper.
My favourite place in Bangkok to eat grass jelly is an old shophouse just a few steps away from Tha Chang. There are a couple of chairs and tables inside, but most people simply sit on the stools set out along Thanon Maharat, as shown in the picture above.
The grass jelly itself has subtle 'herbal' flavour, and is served with crushed ice, but my favourite part is the unprocessed cane sugar, naam taan daeng, that the dish is topped with: