i hope this isn’t too late…
firstly, whatever you wear, make sure you’re comfortable in it. And i don’t mean comfortable as in sweatpants, but comfortable as in confident, that you know you look good in it! Do you have a pair of dark skinnies? they’re the easiest thing to help you look put together…a pair of flats with a great fitting t-shirt and one or two great accessories (necklace? earrings?) would look easy and flattering. Mini skirt and tunic sounds great, but personally i’d avoid plastic/rubber flip flops.
Another thing to keep in mind would be to bring a cardigan or other cover-up, so that you have options.
i hope that helps! and i hope you have fun on your date!