If you have a lot of time but little spending money, simply doing long walks around the city, including residential neighborhoods, can be fulfilling. There is tons of interesting residential design and occasional gems in the various neighborhoods that satelleite outwards from Shibuya.
If you can swing a little spending money on souvenirs, do so. But I'd rather prioritize food in Tokyo over sacrificing a range of food and eating only lowbrow stuff in order to cop goods. Tokyo is about food these days. Not everything is great, but there are a lot of amazing restaurants and dining experiences. Not all are expensive, and there is tons of quality to be had.