If you have the basics, immediately consider the following.
Tripod (consider Manfrotto SILK 615-315: $150)
Features to consider include weight, steadiness, snap-on connection to camera, smooth movement in moving the camera, and adjustability of legs. We prefer lightweight but steady, whereas serious videographers find it worthwhile to lug about the heavy ones. You may prefer spiked feet outdoors, but pads are better inside. Castors are okay if they can be locked easily. Being able to get the camera level, and to swivel it smoothly side-to-side and up-and-down, are important. What are called “fluid heads” are best, but some mechanical ones are satisfactory. Try before you buy.
Microphone (consider 2 lapel mics: $50, and an omnidirectional mic: $70)
The latter can be hand held or put on a stand or a cushion. Medium quality is enough. The built-in mic is convenient and often is enough. But some people, often children, speak very softly, or there’s a noisy environment, or the camera noises are intrusive. So be prepared with microphone options.
Lapel mics are great for individual speech, but using more than one presents problems for self modelers. Adapters can be used to feed to mics into a single jack on the camera—but it’s a fragile connection so you must monitor while recording, with headphones. Wireless speakers help (expect to pay $200 and recharge batteries often), but we’ve experienced interference from radio stations. Bluetooth Wireless Microphones go with camcorders equipped with an Active Interface Shoe. Sony ECM-HW1 for about $150 is a model to consider.
Better, hand-held microphones, have large plugs that probably need an adapter for your camera—again a tricky but manageable connection. You may like to use a unidirectional mic and put it on a tall microphone stand near the camera pointing at the people in the picture and not picking up so much noise off the side.
Headset (anything reasonable you’re comfortable with): $10
Something from your portable CD player or the airlines will do, as long as it fits your camera.
Extra Battery must be the same power-rating/fitting as the one that came with the camera: $30
The camcorder will probably come with just one battery. Another the same would be okay. But do some research on brand alternatives, and longer juice or lighter weight.
Battery type. Currently, NiMH (nickel metal hydride) is most common and arguably best choice, because they can be recharged for up to 2 years. NiCd (nickel cadmium) previously popular because it is light and holds its charge well—but often loses its capacity (misleadingly called ‘memory effect’). Li-ion (lithium ion) may be the choice of the near future: light, high energy, and rechargeable up to 3 years—but expensive.
Carry Bag (anything big enough with a few compartments: $30)
The camcorder manufacturer would like you to buy theirs, but they are conspicuously branded and might as well say “Steal me!” The more expensive the camera, the more dull and tattered the tote should be. It doesn’t need to be padded and reinforced in our opinion, unless you are checking it with luggage on a plane…