There are several options, but I’d recommend an oiled finish. Products like Treatex Hardwax Oil give a natural finish and can add to the visual ‘softness’ of the wood. While oil will not be the hardest-wearing of floor finishes for timber, the great thing is that it can be replenished and areas re-treated without having to sand the whole floor down, leaving no visible ‘tide-mark’. Varnish-type seals may last a little longer, but when they wear it is next to impossible to successfully re-coat without fully sanding.