You can combine the colored souvenir coins you find in different combinations to access powerful traps and items, as noted below (turn in coins in the order indicated for the particular reward you want):

Blue, Blue, Blue - Sentry Gun
Green, Green, Green - Repcrator
Red, Red, Red - Medusa Device
Blue, Blue, Green - Electric Trap
Blue, Green, Green - Boom Box
Green, Green, Red - Sentry Turret
Green, Red, Red - Boom Box
Red, Red, Blue - Fireworks
Red, Blue, Blue - Window Laser Trap
Red, Blue, Green - Kindle Pop