Hi Aishu,
Cost is always a constraint. You can take a nominal contribution of Rs. 25/50/100 depending upon the employee strength and capability. Also, you can pull in some from management. We celebrated a wonderful foundation day with 10000 (number of employees were around 50). We arranged a collage competition, best-dressed male/female, and provided snacks and refreshments. You can also organize a talent hunt, fish pond, rangoli competition, etc., which will cost you less. Giving small tokens like mugs as prizes is a great idea.
For other branches, you can arrange a surprise cake from Monginis or the local cake shops. I am sure you can organize a low-cost Diwali celebration if the spirits are high.