Don't stay there longer than a night, it's a stop-over city with limited interest.
For all departures in the Amazon, be sure to check with your GP to discuss anti-malaria treatment.
Tabatinga is a link city where you will go with the sole purpose of crossing the Amazon. Indeed, it's not a touristic city of great interest.
We stopped off at Tabatinga the day before heading to the Amazon River and preferred sleeping in the neighbouring Colombian city, Leticia. It's not hugely interesting but a 15 minute trip in a taxi is worthwhile. There are a few restaurant, but don't expect a gourmet cuisine.
Upon arriving in the morning, we went to the port of Tabatinga to buy tickets for the largest river in the world: the Amazon. As the trip was only scheduled for the next day, we had the whole day to discover the city. Having quickly gone for a tour, we walked in the city's market located just to the side of the port. If you have time, it's a chance to enjoy the local colourful atmosphere with sales of artisan products and hammocks. Then we had lunch on site and tried Brazilian dishes all in a very intense musical atmosphere.
For an excursion in the jungle, ask at your hotel, they all provide a lot of information on the steps to be taken.