there are no discounts for being a past cruiser

but if you book within 120 days of your last cruise, you should receive a $200 on board credit. i say "should" because that is always subject to change and is not announced if you book far out (e.g., i just rebooked us (after being on the 3/9 "fire cruise") for may 2003...our reservationist said that the $200 isn't available that far out just yet, but she'll keep an eye on it as we get closer to ensure that it is there for us....we should have had one this cruise (we rebooked following our 9/2000 "exhaust cruise"), but somehow it wasn't there...they are currently researching our missing credit).
as for pins...i dunno about that, but i plan to call them to see if they've got newer ones than the last set i received. it wouldn't hurt for you to call the 800# and ask if they'll send you a set.
as for the sports bag...that is the current "return cruiser" gift, but you don't receive it until you actually get on board your second (or subsequent) cruise. we've also received a picture frame as well as a memory book with pins in it, but again, not until we were on board. they have the return cruiser gifts sitting on the end of your bed when you get to your stateroom the first day.
as for the castaway club reception: nothing terribly special...although the quality of the appetizers they put out this time was much greater than in the past. yes, there are free drinks, but our experience has been that they only serve the bahama mama as well as soft drinks. the captain and several of the senior officers attend and circulate so that you can chat with them or ask questions. the captain also usually brings his trading card and will sign them for the kids (and the kids-at-heart).