This!
Also, what's the budget!?
Drill down to what you really want.
If you want relaxation then by far the most relaxing holiday I had with my similar aged child was Butlins. We went on a Mr Tumble break and had entertainment all day long for her, we could relax, have a coffee, had a drink in the evening etc. Everything was set up for her age group, we had to do very little 'management' of her other than offering her snacks.
Haven holiday in Great Yarmouth not quite so much entertainment but also leisurely. There was a morning activity for toddlers every day about 9.30am, another session in the afternoon, swimming pool with nice warm splash bit, kids disco from 5. We would alternate day on site with a day out- boat on Norfolk broads, sealife etc.
If you want an abroad holiday, the only way it is going to be 'relaxing' is if you look into a break that has childcare. She will be too young for kids club- I think hotels like TUI Blue and First Choice holiday villages have a paid for crèche. P&O cruises also have entertainment/ crèche.
We went to Spain not long after we went to Butlins and just stayed in a self catering apartment with a pool. We had a lovely time but it had its moments and it in no way, shape or form could be called 'relaxing'. Child did not want to sleep once out of routine, we were pounding pavements around shuttered up old towns at 6 in the morning as she was up and nothing open etc 😬, we needed the siesta as much as she did in the afternoon!
We had lovely times at the pool and she shared tapas at the plaza with us in the evening, it was enjoyable overall. However when we landed my parents came to the airport and collected her for a sleepover as we were totally shattered!