Basically, just don't indicate it's for marriage, and don't do it in front of two witnesses. Yes there's complicated discussions brought down about mishloach manot, plus discussions about engagement gifts. At the end of the day, Rabbi Hershel Shachter shlit'a is on the record that the "single guy shouldn't give single lady mishloach manot" concern is a chumra that extends beyond normal halachic concerns.
I would be so bold as to propose (not that propose!) something simple: normally, single guy gives single girl flowers, fine whatever. But on a day when many people are feeling silly and everyone is going around giving stuff to their neighbors, we don't advise a single-guy-single-girl practice. Because every now and then, a guy (sometimes the girl too) fills silly and says "you are wed to me" while handing her a Hershey bar, pizza slice, or the like. (This makes a lot of headaches for rabbis.) So when telling people to go do a mitzva, we don't encourage playing with fire!