Oak, bourbon, coffee, roasted malts, cocoa...lots of stuff.
I would split it into small batches, try varying amounts of additions, see what you like. I'd hate to have 5G if it came off as overpowering.
Give a man a beer, waste an hour. Teach a man to brew, and waste a lifetime! Bill Owen quote
Join the Beacon Point (Aurora, CO) Brewclub on Facebook- casual, fun brewing, drinking, socializing!