It's totally amazing what you can do with a good scanner and printer these days. Find a nice Valentine's Day graphic, scan it and print off a pile.
Take a piece of cardstock, trace out the footprint of an envelope, stamp up a design of some kind to incorporate the mailing and return addresses, scan that into your graphics program and print out your envelopes. I even aged my envelopes with some walnut ink then let them dry before printing a batch.
It's just fun, that's all, just fun. If you'd rather cut to the chase, I've got a Vintage Style Valentine PDF in my Etsy Shop that takes all the graphics prep work out of the deal and lets you get right to the FUN.