Cast member lanyards and hip packs are hit and miss but just keep your eyes open and you'll spot them. Boards at a lot of stores - you can always ask cast where other boards are for trading. Mostly scrappers, as always, but some treasures can still be found with luck.
Trading with other guests happens from rope drop to 3pm at the tables in Frontierland. Note: do not try to sit at the benches and trade - cast will chase you away - just stay at or near the tables. Weekends are generally busiest times there. But I've often stopped and been stopped to trade with other guests when I pull my trade bag out to trade off lanyards or boards all around DLR.
You can also check the picnic area on Sundays but with blockouts and events happening offsite, there aren't many people there these days.