A rain gauge. I got my husband one a few years ago and he LOVES it. He has it situated near his car parking space and he checks it religiously every day, and records the reading. He's up to his 3rd year now so he can compare rainfall patterns.
My DH is getting the patio/concrete cleaning attachment for his gerny - i have no idea what to buy him otherwise.
Great ideas- alas he already has both.
The kids bike seat sounds great though!