Surprise proposals are a tough one to style because you never want to make it obvious, obviously. You want them to be photos that you'll love for years to come, but that don't look unlike something you'd wear to a nice dinner out. So to pick your outfits, suggest that you go out to a nice dinner and let them pick the outfits. You want something that's going to coordinate, but not match. For example, you could wear a nice pair of jeans and a plain button up top and they could wear a cute sundress (or similar - time of year will impact this). Just make sure colour wise is in season, meaning don't wear bright pink in the fall.
If your partner is one to get their nails done, maybe treat them to an appointment at the salon just because so they can have a fresh set of nails for those detail shots. Let them get their roots touched up and those split ends cleaned up. Maybe even get the brows tamed!
You want to suggest these things without making it obvious what you're planning, so if you have to, let someone they would normally go to the salon with in on the surprise and have them suggest getting their nails, hair, etc. done together.
On the other end of the spectrum, if you aren't the kind of people to get dressed up very often, asking them to pick outfits like you're going to a nice restaurant may make them suspicious. Instead, plan this surprise engagement around doing something you like to do together, for example, maybe a picnic in the park. You can still dress up in nicer clothes without making it to suspicious. You can still have that friend take them to get their nails done so they still look good and not raise questions.
For more tips on how to plan the surprise proposal, reach out to talk. We can help!