As far as a remote siren, I'd say the Code 3 3000 or 3050 (hope I'm right on my model #s) would be a cinch. If you're willing to do a bit of extra wiring (not much, just a few extra wires), a Unitrol 80k would sound a bit better IMO. Either one, you can use a simple three position toggle switch to cover your basic tones. If one has the need/want for an airhorn, they can install a momentary switch (gets into that bit of extra wiring, but not much at all).
Another easy to install siren that pops off in my head is the Code 3 Scorpion. It has two flying wires from the rear to hook up to the speaker and two spade terminals for hot & ground. This is for the non-PA model. This unit is just the siren only, so you'll have to worry about mounting the amplifier in a console, under the dash, etc. A few Sho-me non-remote sirens is the 30.2330, 30.3104 and 30.3106/9. All three are easy to wire in, but I don't like the Sho-me units that much due to the fact they sound like they belong on a Power Wheel toy car.
As talked about in a recent thread, the FedSig MS4000 is another good unit to try.
Those are ones that just popped in my head as easy to install from some major brands. I'm sure guys will come in with some decent units from Star, Signal and perhaps other brands.