I second Gertrude's opinion: floatplane time would be highly appropriate (better keep a very specific record of such experience for insurance purposes). Time on tailwheel airplanes might be another idea.
Gengis' suggestions are also good (although if you have, or later obtain, gliding experience, you will no doubt have a separate logbook for that).
I don't agree with the suggestion of devoting a column to stalls and spins; you only have three blank columns, so it seems a shame to waste one on something like that (when you practice the relevant exercises, just make appropriate notations in the "remarks" column). But the idea of a cross-country column makes sense (don't limit it to solo; whether the flights were solo or dual will be obvious from the "Pilot in Command" column).
Notwithstanding all of the above, I'd recommend leaving all three of the blank columns 'virgin' until after you've completed the PPL, when you'll have a better idea of what sort of flying you'll regularly be doing in the future.
One final comment: I've seen too many logbooks that are a complete mess. As your logbook is the only permanent record of your flying, keep it nice and tidy: always print neatly, and use only one colour pen (IMHO black usually looks nicer than blue, but that choice matters less than being consistent).