I haven't done anything like as complicated as a spreadsheet. I tried a couple of garden planning programs and got quite frustrated with them. Now I have a notebook with squared paper on which I've drawn my beds. These I use to plan how many plants and where they're going to go in each bed. I can plan for individual cabbages, say, or a patch of carrots. This book also contains rough dates for lifting crops and details of the crops to follow.
I've also indicated in the book how much (if any) muck and/or ash each bed requires. This has been handy as my oh has been doing my rotavating this year and he can take the notebook out with him and easily follow it. In fact, this has perhaps been the most useful feature of the notebook this year. Buying a software program may prove an unnecessary investment because after a few years, you soon get an idea of what, how much and where you want to grow things. This is only my second year of what I laughingly call my "grand plan", but already I have a feel for roughly how many onions I can grow, for example, where they'll go and what space I can give over to legumes before the overwintering alliums go in.
Personally, I like having this in a book as I find it more flexible. I can ponder on it when the computer isn't switched on and when I take it out with me, making it easier to compare with the real situation on the ground. For example, while putting in some lettuce plants the other day, I spotted that my oh had slightly widened one of the beds and instead of having a single row of cabbages, as I had planned, I can now have two staggered rows, making it possible for me to grow more or freeing up space elsewhere for something else. For example, where I had planned to grow cabbages, I can now grow something else in the same rotation group instead, such as turnips. You can of course take a print-out of your computer plan with you, but even though I'm relatively tech-savvy, I like to have the durability of a notebook and to be able to update it instantly and on the spot rather than having to go back and switch on the computer.
As for sowing times, I've got a month-by-month schedule in Word. This could perhaps be better organised in a proper calendar program, but as I can't plan for particular days, only months (and even then things sometimes slip), this Word document is satisfactory for my purposes and it couldn't possibly be easier to organise.
It may be that this works for me because my main veg patch is relatively easy to organise: seven beds of 15 m long, five beds of 12 m long, all aligned east to west, making twelve beds in total, which I have divided into four rotation groups of three beds each.
I also have a somewhat more complicated smaller patch which has given me some headaches this year. Each year a bit more land comes into cultivation in this patch. It is more sheltered than my main patch and I work it very hard for climbing or delicate plants in the summer and for my overwintering crops too (spring caulis, for example). Even so, I still use my notebook and Word schedule for this.