Wheels are probably a good way to go. You can generally use wheels designed for 11 speed with 10 speed cassettes by using a 1.85 mm spacer along with a 1 mm spacer. Check the specs to verify, though.
Depending to the groupset the bike currently has and how many miles it's seen, a partial groupset upgrade can be nice. IMO it's only worth it if you're willing to do the work yourself and can find components that are a genuine upgrade from what you've already got at a decent price.
None of this will make the bike any faster. It can make it more fun to ride, though - and if you end up riding more, then you're likely to get faster.