Hi Melorious,
If you are going on an all inclusive trip you shouldn't spend much money at all. Souvenirs, drinks, extra outings (of which the hotels in Mombassa have many and know how to charge for them too).
Actually if I was to stay in Mombassa again, I'd get to know a taxi on day one and tell him how much you're willing to spend, if it's too little for him he'll quickly recommend someone to fill his shoes. Once you have a driver you can decide what you want to see and exactly how much it will cost you. The taxis outside your hotel will be expensive but be firm and let them know that you know they'll charge too much.
The 'Beach Boys' in mombassa have some interesting ways of making money and one of them is a Beach Safari where for small money they'll show you the creatures and plants living in the vast rock pools when the tide is out. A really cool and interesting few hours I guarantee, plus you're putting money directly in the hands of the locals.
Tipping is at your discretion, if it's deserved give something small, if it's not deserved, don't. The average salary is about $30 a week for hotel workers, and that's quite high so you can work it out from there.
When I've been on Safari before I try to judge how much of my experiences were down to the driver/guide before tipping. A good guide is worth every penny he gets. I've seen guides who go through the motions with tourists only pointing out the very big and the very obvious animals but if you happen to get one who shows you the small stuff like bat-eared foxes, civets, gennets, porcupines, snakes and all the hundreds of bird varities and knows all about them, tip well, they've learned their stuff.
If you have any specific quesitons feel free to PM me.
As for staying in Nairobi Stephy.
You can find every level of price range for accommodation in Nairobi from 1Euro per night to the rip off hotels charging 300/400 per night.
I have been hearing great reports about Wildebeest Camp in Nairobi. It appears to be a very well run, very clean hostel/hotel/tented village right in the affluent part of the city. The prices on their website look reasonable too, starting around E15 per night for a single room.
As I said I've never used them but I certainly will next time around.