I like the idea of the 15-85 (considering upgrading from the kit lens myself), but regarding band for buck and versatility, the 18-55 plus the 55-250 might be an alternative. Not fast, not very sharp wide open etc, but not bad and more reach (if that is a requirement at all).
If off-camera flash is an option for the near future, the T3i can trigger an off-camera speedlite.

Arnt