I highly recommend a flash. The hard part is choosing which one. The 580EX II is the caddy of the Canon speedlights, both in terms of power and features (namely the ability to act as the master flash that can control up to two groups of slaves). The 420EX is a close runner up, not as powerful, and slightly less features, but still an outstanding flash. The nice thing about the 580EX is that it has a PC port on it, so if you want to trigger it remotely using Pocket Wizards, or more cheaply,
EBay triggers. Both can rotate for bounce.
There are other "off brand" flashes which are cheaper, and offer a lot of the same features, such as the Sigma EF-530.
If you don't care about E-TTL and just want full manual, it gets really cheap, such as the Vivitar 285HV