Where can you fly from? Have you got a budget in mind? Do you want guaranteed heat / sun or happy with mixed weather?
Some initial ideas:
Copenhagen - good public transport, very safe, could do a day trip to Malmo if going for longer
Switzerland - any of the Swiss cities really, or could combine two if wanted, Geneva, Zurich, Basel, Bern, Lucerne, Lausanne etc. all very safe feeling - lakes are beautiful too if wanted more of a water based holiday
Ljubljana - good value for money, lovely city and not too big, very safe feeling
Austria - again could combine two cities or just do one - Vienna, Salzburg, Innsbruck - or the Austrian alps if you’d prefer a non-city break. Great public transport.
Warsaw / Krakow - either is great value for money, always felt very safe
Valencia / Malaga / Alicante / Palma - all lovely Spanish cities with beaches and good public transport
Trieste - one of my favourite Italian cities to travel solo, very safe, nice beaches, good public transport