Travelling Scotland without car

I live in the North East of Scotland (do the Math) and want to explore. My biggest drawback is I do not have a car/drivers licence, so most places people say to travel to are impossible to do relying on only public transport. Any recommendations would be massively appreciated.

The sort of things I am interested in/have on my bucket list: Wallace Monument in Stirling. Any form of Castle/Castle Ruin or abandoned buildings (I’m aware it’s extremely frowned upon to give out that information, I’m simply using as an example). Basically anything that has some form of history and/or importance. Small hikes and forest-y areas are something right up my alley. - However suggestions are not limited to that whatsoever, anything is welcome.

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The only “no, I’m not doing that” things are the basic suggestions in Aberdeen (shopping, pubs, nightlife) or Peterhead Prison (been there enough times where I don’t care about it). Aden Park in Mintlaw, that’s already on my schedule, and Glasgow and Edinburgh. This is because it’s not fitting for a day trip, in my opinion.

If any tips or recommendations take price into consideration, even better! I’d cap my budget at about £200, excluding trains.

Thanks in advance for those who respond!