Shame you’re not bringing bikes as there is some epic cycling to be done in West Sussex – although more road & gravel than MTB.
I live in the South Downs Nat’l Park (near Petworth) and the road and gravel riding around here is epic. You are within 20mins of the beach if you want, have excellent towns like Midhurst, Chichester, Arundel, Brighton, Worthing, Shoreham etc within easy riding distance.
You also have lots of interesting things/places and stuff to see… such as Arundel Castle, Petworth Park, the Weald & Downland Museum, QECP, the Centurion Way, Goodwood, the Roman Villa at Bignor, Iron Age hill forts up on the South Downs Way, the Downs Link from Guildford to Shoreham, Knepp Castle/wilding, numerous excellent vineyards (Nutbourne, Nyetimber, etc), beachfront cycleways all along the South Coast, excellent cafe’s/cake stops and of course all very enjoyable in our tropical, balmy weather down here… (ok, I’m beginning to sound like a travel guide!)
Bring a bike.
Happy to show you around a bit if you decide to come.