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If you want really good value for money, a season ticket at Paradise Park is £35 and admits two adults and up to three children. The themed garden and play area is good in the summer and the exhibits, soft play area and amusements are good for winter. My boys never seem to tire of the place, although after the 20th visit you may lose the will to live.
Friston Forest is only £1 parking.
Railwaylands is free and you can walk there.
Book a train ticket at least seven days in advance on Southern Rail's website and you can travel anywhere in the region for £10 whilst kids go for £1 each.
Birling Gap has free parking and rock pools when the tide's out.
Baxter's Field is free and usually empty - perfect for picnics and ball games (but no barbecues or dogs).
Brighton's Sea Life Centre is ridiculously expensive, but if you book on line it's much cheaper.
Join the Sussex Archeological Society and you can visit the Castle and Michelham Priory as many times as you like.
Wilderness Woods is about £33 for a season ticket - there are lots of activities for kids and you can make dens, have barbecue picnics and even camp there.
I'm absolutely skint, so I've become an expert on cheap days out with the kids and they don't seem to be any happier when you spend a fortune on them.