Charger and spare battery. My SL-C700 review notes the poor battery life (approximately 4 hours with low backlight, less if you listen to mp3s or use a WiFi card). One strategy might be to carry a spare charged EA-BL06 battery with you. For this you would need a CE-BC21 battery charger. Its specifications are: Input AC 100-240V 50/60Hz, Output DC 4.2V 650mA, Dimensions 65mm 80mm X 26.5mm (or 2.56" X 3.15" X 1.04"), and Weight 75gm (2.25oz). Note: you use this to charge an extra battery, which you would (quickly) swap in.
SL-C700 and GPS: SDG Systems sells Locatus, a CF GPS which works with the SL-C700. Go here for details...
CF modem: Picked up a Targus modem, model PA560U at CompUSA for $29.99. Popped it in , set up a Dial PPP account and it worked right out of the box. Go here for more.
Microphone: The Voice Recording application will only record with an external microphone.
Extended Battery. If you purchase the BL-08 battery kit you can put it in your SL-C700 and increase your autonomy with the 1700 mAh battery.