I love bank holiday weekends - i got so much making done! I had a chance to try out these pretend envelope liners, where you just stamp the inside of the envelope instead of adding a liner. The idea is from Oh Happy Day. Just tape off the edges of the envelope you dont want to stamp. I used low tack masking tape, id recommend you try a area first to check it peels off ok and dont leave the tape on too long. Then just start stamping, the more random the better. I used the rubber end of a pencil for my stamp and some little ink pads i got cheap on Ebay. Use a thick good quality envelope though as it'll bleed through thinner paper.
You could also make an actual liner with this stamp patter too. Kids tracing paper makes a great envelope liner and is relatively cheap. Just cut it to size and stamp away. Use a bit of temporary spray adhesive to keep it in place (the permanent stuff doesnt give you a chance to push the liner in very easily)