I do love swimming toys! Swimming is relatively cheap in terms of equipment in comparison to other sports. Swim cap, togs and goggles and your off. However there is plenty of tools or toys as I like to refer to them should you be interested.
The first thing I can remember purchasing all those years ago was fins. Speedo biofuse fins are an excellent investment which can be purchased via amazon for around 20/30. They will help with your body position in the water and strengthen your legs. Also its a lot of fun with fins on as you swim much faster!
The next purchase I can recommend would be hand paddles. There are many versions of paddles out there, from technical paddles to power paddles to some I dont know the name of. The main benefit being they will help improve strength within your lats and tri-ceps.
Have you mastered the use of a centre snorkel? I havent! Tried many times but still find it difficult to manage my breath using this tool. However for people who can easily do this I can appreciate that not having to breath by turning sideways means you can focus on executing a good stroke.
Bungee cord – tried this a few times at uni with a friend who had recently purchased one. Endless fun trying to increase the distance you can swim before the tension gets too much and you get thrown backwards faster than you would believe.
Parachute – never tried it, @finis I would like to try this if you have any spare?
Pool-mate Pro – fantastic swim watch, measures everything. Highly recommend!