I live across the road from the Park and walk there every day.
The highlight is the view up the Lake to the fells. The best time is at sunset, when the light is lovely and the park is quiet.
The parkland area is pleasant, particularly in the spring where there are great drifts of daffodils. There is a path along the lakeside, a choice of food outlets, a play area, boat hire, and access to the lake for launching canoes and paddleboards. The swimming is delightful, but watch out for currents when the lake is high.
If you are local, there is a sailing club, a rowing club and a canoeing club at the Activity Base.
There is also a pleasant walk through the meadowland to the south of the park that most visitors miss.
If you are a member of the NT the parking is free, but it's quite an expensive visit for everyone else. Don't be tempted to park on the road outside - the police are clamping down and you may well get booked.