Here is a list of my recommended useful accessories that I have bought for my MP4 Player.
1.) USB Battery Box
If you bought an MP4 player from Hong Kong you will have noticed how short the battery life is when it has been fully charged.
One useful gadget I found is a USB Battery Box. I bought two types. One that takes 4 x AAA batteries and the other that takes 4 x AA batteries.
My MP4 player lasts just over 1 hour when fully charged and was frustrating. Especially when I put a 1 hour 30 min film on it.
With the 4 x AAA USB Battery Box using NiMH 800mAh rechargeable batteries the player now lasts approx 3-4 hours and with the 4 x AA USB Battery Box using NiMH 2100mAh rechargeable batteries the player now lasts approx 6-8 hours.
The AAA USB Battery Boxes dimensions are H:70mm W:50mm D:20mm approx and the AA Battery Boxes are H:130mm W: 35mm D:25mm approx.
I personally prefer the 4 x AAA because of the size and fits into a jeans pocket better and both also have a belt clip on them.
These USB boxes are also compatible with any device that uses power from a PC 5Volt USB port, like Mobile Phones, PDA's, Personal Cooling Fans, Reading Lights Etc.
2.) Car Cigarette Lighter USB Adaptor
There are several types but the main 2 types are an adaptor with 1 USB Port and another with 4 USB Ports.
Again these will also power other devices that use power from a PC 5 Volt USB port.
3.) 2.5mm to 3.5mm Headphone Jack Adaptor
The headphones that came with my MP4 player were very poor and tin sounding with crackles. When I went to plug in my stereo headphones I then realized that the headphone socket is for a 2.5mm jack and not a standard 3.5mm jack.
There are 2 types of adaptor:-
2.5mm to 3.5mm Plug
2.5mm to 3.5mm Lead.
They are also available in white or black colour.
The adaptor I would recommend is the lead type as the plug type pertrudes out of the player and in time wouldn't take much to damage the players headphone socket when knocked.
The difference in sound quality is really noticeable and now any headphones are compatible. No need to worry about finding somewhere at short notice that you can buy a pair of headphones from with 2.5mm Jack. Especially when the supplied ones break after a week or have only 1 side working. The supplied headphones I had have to be the worst ever, but amazingly they did still work after a week before I received the adaptor. The £ shop sell better ones LOL. Seriously!!!
4.) 3.5mm Splitter
This is a useful adaptor so that 2 headphones can be plugged into an MP3 / MP4 player that only has a single socket. There are 2 types which are a plug and a lead and are also generally available in black and white colour.
Please note that there are some poor quality types of these adaptors and reduce the sound volume when split between 2 headphones so it is worth buying a higher quality one like Belkin.
Hope this helps out and without a doubt will get you the most out of your MP3/MP4 player.