I need Ideas and themes for about 20-30 people, also need food ideas and etc.
Here’s a link to get some ideas for themes. You can do a “dress from the past” theme or a “tropical” theme. For a tropical theme…you can decorate with bright festive colors, have a colorful cake, barbecue/grilled meat, fruit salad in a basket made from fruit. I’ve even seen people stick tall colorful lollipops and candies in cut pineapples and melons for parties and it looks really festive and cute. You could do a historical time period theme…fancy dress theme…princess theme…Arabian theme with arab food, etc etc. How about a Mughal time period theme? It would be fun to have the girls dress up in jewelry and clothes (anarkali dresses) from that time period. Or you could base your theme on a particular movie. If the guests are mostly desi…you could have a desi theme. If they are multicultural…you could have a more broader theme so everyone can participate.
General - Sweet Sixteen Themes
For food, there are so many ideas:
Food:
chicken wings
Pizzas or flat breads
bread wedges with hummus
chips and salsa
potato skins
jalapeno poppers
shrimp cocktail
potato cutlets
fruit platter with dip (buy pre-made from grocery store)
veggie platter with dip (buy pre-made from grocery store)
samosay/pakoray (if you want desi food)
egg rolls
stuffed mushrooms
mini quiches
cheese and crackers
tapenade
Sliced bread with various spreads (spinach artichoke dip/cheese fondue, relish)
Finger sandwiches (egg salad/chicken/cheese and cucumber)
Creamy salads (egg salad or pasta salad)
chocolate fountain? With an assortment of dippers such as pretzels, biscott, fruit, marshmallows, bread.
Cookies
Cake
Candy
Ice cream…ice cream sandwiches
Brownies
Brownie a la mode
root beer floats
desi deserts (ras malai, mithai, kheer, etc)
Fruit punch, soda, pina coladas (yum)
Re: Sweet 16th Birthday Party
you could do something really sophisticated and pretty too with colours and decor, in terms of a theme, rather than do a dress up party.
chocolate brown and pale pink would make a great combo- you can look for inspiration here- Style Me Pretty - The Ultimate Wedding Blog
i know that site showcases weddings but they have lots of great ideas and DIY projects you could do, and tons of inspiration for really any kind of party that you could spin from someone else’s wedding decor. plus, it only has to cost as much as you want 
Re: Sweet 16th Birthday Party
i have a better option (solution):
you can not throw one! :D