The biggest problem with the shin/knee pad combos is that they just don't stay up, especially when surfing. I found two ways around this issue: 1) Wear your pads between two wetsuits; or 2) Buy some marine quality velcro and some contact cement. Cut off a piece about 4" x 2". Glue one side to the inside of the top part of the knee pad, just above the knee. Glue the other piece to your wetsuit, where the two shall meet. Bingo, no more pads slipping down.
I use the hard plastic kind for mountain biking from 661, although they make me look like I work for Lord Vader.
Hands, Elbows and Feet
I use Psycho O'Neill surfing gloves for cold water, & webbed gloves for warmer water. In the warmer water, gloves can prevent chafing against the hard parts on your boards, like the screws that hold in the straps.
Elbows can get somewhat beat up when creeking. I use rollerblading elbow pads, but kayak shops carry pads that cover more area and stay in place better.
Some like booties with a hard sole; I prefer fin socks. Hard soled booties make it easier to hike the trail, scout, & portage, but you'll need bigger fins. Fins will chew on your ankles and toes, so if you don't need booties for warmth, wear them for comfort. As the thickness of your bootie increases, add a couple sizes up for your swimfin.
Elbow rollerblading pads; knee/shin combo pads by 661, O'Neill DL Psycho gloves