Littlehampton is a great place to go for the day. It only takes about 45mins to get there from Guildford.
You can enjoy a day on the beach, or, if the weather is not too warm, you can walk along the promenade, walk to the harbour or spend a bit of money at the amusement park.
There is a good play park along the seafront and children enjoy taking the small train along. Back from the harbour area there is a pond with pedal boats for hire.