We went last year with our then 22 m.o for 5 days.
The beach is lovely and if the weather’s good you can have a few bucket-and-spade days
The aquarium is good fun and has a little playground and splash park
Walk around the old marina looking at all the boats, and out to the pier. Some nice pubs and bars round there too.
You can hire buckets and nets and try your hand at crabbing
The Sambo Rodizio restaurant is a little bit of a walk from the town centre but well worth it for the food! Otherwise there’s plenty of fish and chip shops to take your pick from. Mona’s Thai restaurant was also pretty yummy, we had a takeaway from there one night.
Bear in mind you will absolutely need to book ahead if you want to go anywhere for dinner and probably lunch too!
Lulworth Cove is a not too long drive away, as is Corfe Castle which has a gorgeous little village you can walk around, a model village and a train station where you can get a steam train to Swanage.