You could easily spend 7 days exploring the Mt. Kenya region. Perhaps do the 4 day Mt. Kenya trek and then use the other 3 days for safari in Ol Pejeta and another activity? In the Mt Kenya area, you can do day hikes on Mt. Kenya, day hikes in the neighboring Aberdares, safari in Ol Pejeta, white water rafting, mountain biking, and there are fun bars and restaurants in Nanyuki town.There are also a lot of budget and mid-range accommodation options near Nanyuki that can help you organize any activities. That being said, though, the Masai Mara is really incredible and while there aren't too many options for physical activities, the safari experience is unbeatable. In my opinion, climbing Mt. Kenya and seeing the Masai Mara (unless you fly between locations), might be a little too much for 7 days, especially since you need to account for getting in and out of Nairobi. Perhaps, instead, you could spend a few days in the Mt. Kenya region doing day hikes/mountain biking/rafting, then spend a night or two in Naivasha (where I highly recommend spending half a day in Hell's Gate where you can bike through the park and take a really nice and fun gorge walk), and finally spend a 2-3 nights in the Masai Mara. You can find fun campsites and cottages in Naivasaha for a reasonable price and there are many budget/mid-range lodges and campsites in the Mara. Hope that helps!