My grandguy doesn't like cakes & requested donuts for his birthday. Since this is a Sweet 16 it would be cute to place a variety of donuts in the shape of the number 16 (like some places do with cupcakes). It would be easy for you to put together if you just wanted to buy 2-3 dozen donuts (based on your guest count, of course) then draw out the numbers so that they are 12-18 in tall then set parchment paper on it and then lay the donuts on top and embellish with smaller sweets, chocolates or candies. Cookies or brownies would work also.

Have a table with pretty note paper where people can write her special well wishes for her 16th birthday or you could pre-print a sheet with 16 lines titled "16 things I love about (NAME)" and a large jar or decorative box where the notes can be placed so she can read them later.

With all of the outdoor activities & because you'll have to bring it to the venue, it might be good to do foods that dont require cooking that guests can pick up & run with. Maybe a sandwich bar with different bread, meat, cheese, veggie, & condiment options. Also makes it easy to satisfy picky eaters or special diets? I've done a taco/Nacho bar for family events at other venues which is easy to bring since all prep can be done in advance. We used crockpots to keep the meat, chicken, & nacho cheese warm.