I haven't seen it mentioned yet but I'd suspect that plywood won't
wear very well. I think you'll find the intermixed grain in the plies
will wear differently and leave you with lines and dips. The only way
to resurface will be taking the whole thing to a drum sander.

I've seen a lot of benches made from 2x4 studs and they've worn pretty
well. Mine was made with recycled wood. The wood came from a
subdivision that was being fitted with utilities. When steel pipe is
hauled in for sewers, it's racked with 4x4's of different species. A
few minutes with a power planer to clean off tar, rocks and dirt is
all it takes to get some usable wood. Then it's a matter of sizing,
squaring and glue up.

I'd also recommend putting a square of old car/truck tire under each
leg of your bench. It keeps the bench from vibrating back when you
pound on it. It's so much nicer to use a bench that absorbs impact
and responds like dead weight. Wood legs on a wood or concrete floor
will vibrate.