I use a handheld unit. Delorme Earthmate PN-60. It's waterproof, shockproof, full featured, runs off a pair of AA batteries (alkaline, lithium, rechargeables, any type) or 12V cord or AC cord. I have it mounted w/ a RAM mount. I use it on all my bikes, moto or peddle, kayaking, hiking, skiing, geocaching, etc... Has topo (military grade topo), street, nautical, satellite maps... Delorme has the best maps in the industry... Mine has 3.5GB on internal memory (up to 16GB now available), has standard SD card slot, you can add another 32GB of memory using an SD card.
Link to DeLorme:
http://shop.delorme.com/OA_HTML/DELibeCCtpSctDspRte.jsp?section=10460