It's possible to get transport to Namanga (border) and on to Arusha - and enjoy the scenery, even great views of Mt. Kilimanjaro and Arusha (weather dependent, of course), animals, etc. I'd avoid the matatus, but there are other forms of land transport (bus, vans, etc.) and theycan often be arranged on site by competent agents in a hotel or agency. And I think it's Impala shuttle that does the Nairobi - Arusha trip; figure a half-day.