Get a SunPak RD2000 Flash for walk around fill flash and a bunch of eneloops. I'd recommend the Canon 90EX (from the EOS M) over the Sunpak, a little bit more cash, but down the track you can use it as an optical off-camera flash trigger for the EX series - works brilliantly on my 6D for that purpose
The 90EX is larger, twice the price and doesn't pivot which is critical to me. I also have the StoFen diffuser on it all the time. I have some very detailed reviews up on Adorama, B&H, etc. You can get a small RD2000 with the diffuser for less than $100.
I think you're thinking of the 270EX... the 90 is a little smaller, cost me ~$100, while it doesn't have a diffuser or tilt, it's light, and i find the wireless E-TTL a big advantage when using with my 560/600EX flashes