What do you mean by 'chill out?'
Does this mean beach time? Well, if so... it's either Kenyan or Tanzanian Coast.
If this means 'city' time? Nairobi in Kenya doesn't really need more than 2/days, nor Arusha in Tanzania.
With two weeks, you have sufficient time to visit/safari a bit in both countries. And maybe even a few days on the coast. Also, regardless how close these two countries appear on a map, distances can be great and roads not the best, so you have options to 'drive' some places, or 'fly' in small planes elsewhere.
As to this being a surprise, it won't be once the participants have to visit doc for malaria meds and any inoculations needed depending on where visiting, and for sure once they have to start packing and know they're limited in weight. Visas will be required for both countries though these can be purchased on arrival.
Know that Oct is peak season at both countries, so at the highest prices.
Important for you to know yours and their interests and what areas would best suit. This is your holiday, so if you haven't yet done so, suggest you pick-up a guidebook for either/both countries to have an idea of what is on offer... history, culture, game, various parks/reserves, which beaches, weather, health, type of accommodations, estimated costs, etc.
In otherwords, we need more info from you to determine whether you're even on the right track.