there are bungalows inside the park. You should be able to book them through the park website - I just turn up if I go there, so I'm not sure but generally that is the case. The price is higher on weekends and public holidays. There are also a couple of campsites but I guess those are out if you aren't able to camp at Kaeng Krachan. The problem with the campsites is that they aren't anywhere near the restaurants (as in, many kilometres away from them). The bungalows are reasonably close to the restaurants.
There are loads of hotels and guesthouses lining the road right before the park though. You need to pay each time you enter but if you only have a couple of days that's not a big deal. From the gate you can just hitch to the park HQ - lots of buses and trucks go through because the road through the park goes to various villages, and the guards at the gate will flag down vehicles for you.
A possible alternative might be Doi Inthanon in northern Thailand. You can get from Bangkok to Chiang Mai on an overnight bus, and reach Doi Inthanon that morning. Spend a couple of days there, then bus back to Bangkok. It would depend on your parents willingness of course. But the birds up there are different to what you'll be seeing at Kaeng Krachan. And Chiang Mai is a nice city if you have time to look around a bit.
I'll have another look at trying to book the bungalows, if necessary I can ask someone in Thailand to give them a call domestically which will probably be easier. Camping is not a possibility unfortunately because my parents will not allow me to camp by myself and they will not camp with me, which leaves me a bit stuck. A shame, as I would quite like to camp for a few nights.
About the staying outside Khao Yai, I was under the impression that if you stayed outside the park, you could not be in the national park after dark at all, and I would quite like to do a bit of spotlighting.
I originally discounted Doi Inthanon for being too far, but I'll have another look at the logistics of it at the weekend.
Thanks for the advice/suggestions.