Edinburgh is one of the best-placed cities in Britain for a day trip. Within an hour by train you can be at a Renaissance castle in Stirling, on a wide surfing beach in North Berwick, or in Glasgow's West End. Give it a full day on a coach and you reach Loch Ness and Glencoe. This is my pick of the 12 best, split honestly into the ones I'd do myself by train and the ones worth handing to an organised tour, with real journey times and return fares for each.
The Lowland trips are the easy wins, and they're where I'd start. Stirling, Linlithgow, North Berwick, Dunfermline and Glasgow are all direct-ish trains under an hour, off-peak returns are single figures up to about £17, and you travel on your own schedule. St Andrews needs a short bus leg and Rosslyn Chapel a slow local bus, but both are still cheap and simple. None of these need a car, and most work as a relaxed half-day if you'd rather be back in the city for dinner.
The Highlands are the opposite, and it pays to be honest about that. Loch Ness sits roughly three and a half hours from Edinburgh each way, so the only sensible way to see it in a day is a long coach tour, about 12 hours and from £56, which throws in Glencoe on the way. Two practical notes before you book anything: buy train tickets off-peak or in advance, and use the code GOOD25 for 25% off Historic Scotland castles when you arrive car-free by train or bus. If you're doing several castles, the Explorer Pass pays for itself fast.