There is only one entrance to the park, located at the northern end, near the main road and on the outskirts of the nearest village, Mto wa Mbu. (Note that Mto wa Mbu is lined with wall-to-wall curio stalls, the density of which may tempt you to stop -- don't; there is unpleasant pressure to purchase, and you'd be well advised to hold on tight to your belongings, as there is a slightly menacing atmosphere. This is not the case at most other roadside stalls).
Direct from Arusha, the road trip takes around 2 hours; Tarangire's main entrance lies just more than an hour away. Entry is $35 per person, per day ($10 for ages 5-16; under 5 free). Given the small traversing area and heavy tourist traffic it experiences, I'd strongly advise you to overnight within easy striking distance so that you can aim to be there when the park gate opens at 6:30am. As is always the case on safari, the early bird is well rewarded, not only with potentially great game sightings, but also with the peace and privilege of having the park to yourself -- a notion that will be dispelled from 8am onward, as the post-breakfast hordes descend from the escarpment lodgings. Better still, spend the night in the park; this way, you have the park to yourself in the morning as well as late afternoon, by which time traffic is dispersed from the southern end of the park. Note that the park is one of the first in Tanzania to offer night game drives and walking safaris. Green Foot Print Adventure (www.greenfootprint.co.tz) has a license to offer both these activities; inquire at the park entrance or book through Serena Lake Manyara Lodge. The park fees for night game drives are in the range of $50 per person, with a further $50 per person for the park vehicle (this reduces depending on number of guests); walking safaris cost $20 per person, plus the group or individual fee for the guide, at $20.
Generally, the northern section of the park sees the most tourist traffic and, from a landscape viewpoint, is the least rewarding. Make sure you get as far south as the Maji Moto hot springs, where waters, having circulated deep in fissures below the earth finally resurface at 60°C (140°F; hot enough to boil an egg) and provide ideal breeding temperatures for the algae the flamingos feed on. Flamingos aside, the views of the floodplains are well worth the journey (if you like huge barren landscapes), but if you don't have the time to travel this far south (and, preferably, overnight at &Beyond's Tree Lodge), you can exclude Lake Manyara from your itinerary without serious regret. Certainly, there is no point in seeing the park as part of a package tour, where you will be wheeled around to all the predicable points, probably at the same time everyone else is.
Note: This information was accurate when it was published, but can change without notice. Please be sure to confirm all rates and details directly with the companies in question before planning your trip.