Flat unfolded cards and traditional card blanks are great if you’re on a budget or don’t have huge amounts of information to include. Wallets are great for including lots of extra information inserts, RSVP cards as well as the main invitations themselves. There are also in_between options that give you the freedom of a wallet on the budget of a traditional card.
Use a neutral colour for the cards themselves, then add colour through printing, embellishments or ribbons, or use coloured card and make a great statement. Card textures can make a big difference to the overall feel of an invitation, from rustic matte textures to contemporary pearlescent effects to vintage tapestry styles. Consider printing or foil stamping the front of the cards, or use peel_off stickers (with a steady hand!).